Light-emitting element, method for manufacturing same, and light emission method

ABSTRACT

An organic EL element ( 10 ) includes a first light-emitting layer ( 33   a ) having the shortest emission peak wavelength of a light-emitting layer ( 33 ) and containing a host material and a TTF material or at least the TTF material, a second light-emitting layer ( 33   b ) containing at least a TADF material, a third light-emitting layer ( 33   c ) having the longest emission peak wavelength of the light-emitting layer ( 33 ) and containing at least fluorescent material. The excited triplet level of the TTF material is lower than the excited triplet level of the TADF material.

TECHNICAL FIELD

The disclosure relates to a light-emitting element, a method formanufacturing the same, and a light emission method.

BACKGROUND ART

In recent years, a self-luminous display device using a light-emittingelement (EL element) employing an electroluminescence (hereinafterreferred to as “EL”) phenomenon has been developed as a display deviceinstead of a liquid crystal display device.

The light-emitting element employing the EL phenomenon can emit light ata voltage from several volts to several tens volts. The light-emittingelement is a self-emitting element. Therefore, the light-emittingelement has wide viewing angle and high viewability. Further, thelight-emitting element is a complete solid element having a thin-filmshape. Therefore, attention is attracted from the viewpoint of savingspace and portability.

Since the light-emitting element is a surface light source, thelight-emitting element is considered to be applied to a backlight of aliquid crystal display device or a light source of an illumination.

In recent years, the research and development of the light-emittingelement employing the EL phenomenon have been actively made. Thelight-emitting element has a configuration in which a functional layercontaining at least a light-emitting layer is provided between positiveand negative electrodes. The light-emitting element emits light by usingrelease of light during deactivation of excitons that are generated byinjecting electrons (e⁻) and holes (h⁺) into the light-emitting layerand causing recombination.

To realize a high-resolution EL display device as a display device usingsuch a light-emitting element, use of a white light-emitting element iseffective. In the white light-emitting element, vapor deposition byselectively patterning vapor deposition using a fine vapor depositionmask is not needed.

CITATION LIST Patent Literature

-   PTL 1: JP 2014-241405 A (published on Dec. 25, 2014).

SUMMARY Technical Problem

In the white light-emitting element, a white tandem structure is widelyused. In the white tandem structure, a plurality of EL elements are usedat multiple states to achieve white light emission. The white tandemstructure has problems in which the drive voltage is high and theefficiency is decreased due to carrier loss in an intermediate layer.Further, the white tandem structure has many layers and lowproductivity.

There is a light-emitting element having a white structure in which red(R), green (G), and blue (B) light-emitting layers (i.e., redlight-emitting layer, green light-emitting layer, and bluelight-emitting layer) are simply layered. In the light-emitting element,it is difficult to generate excitons over the red, green, and bluelight-emitting layers and emit lights of three colors efficiently.Therefore, it is useful to use transfer of energy of excitons(excitation energy) to illuminate an adjacent layer. However, in thiscase, there is a need for overlapping of the emission spectrum of anexcitation material with the absorption spectrum of an adjacentmaterial. The energy transfer occurs only by a specific combination ofthe materials.

FIGS. 10A and 10B are each a view illustrating a problem of a knownlight-emitting element in which red, green, and blue light-emittinglayers are simply layered.

In a light-emitting material, a ground-state (S₀) molecule absorbsoptical energy to cause a transition from a Highest Occupied MolecularOrbital (HOMO) level to a Lowest Unoccupied Molecular Orbital (LUMO)level in the molecule, and excitation occurs.

An excited state includes a singlet excited state (S₁) in which the spindirections in the HOMO and the LUMO are parallel and a triplet excitedstate (T₁) in which the spin directions in the HOMO and the LUMO areantiparallel. Light emission from the singlet excited state is calledfluorescence. Light emission from the triplet excited state is calledphosphorescence.

Energy in a singlet excited state of a light-emitting material thatexhibits blue (B) light emission (excited singlet level: hereinafterreferred to as “S₁ level”) is represented by S_(1B), energy in a tripletexcited state of the light-emitting material (excited triplet level:hereinafter referred to as “T₁ level”) is represented by T_(1B), the S₁level of a light-emitting material that exhibits green (G) lightemission is represented by S_(1G), the T₁ level of the light-emittingmaterial is represented by T_(1G), the S₁ level of a light-emittingmaterial that exhibits red (R) light emission is represented by S_(1R),and the T₁ level of the light-emitting material is represented byT_(1R). In this case, the energy level of each of the red, green, andblue light-emitting materials is increased in the order ofS_(1B)>S_(1G)>S_(1R) and T_(1B)>T_(1G)>T_(1R), as illustrated in FIGS.10A and 10B. Therefore, the energy transfer from the blue light-emittinglayer to the green light-emitting layer and from the greenlight-emitting layer to the red light-emitting layer is likely to occur.

Accordingly, when excitons are generated in the green light-emittinglayer, energy is transferred from the S_(1G) level of the greenlight-emitting layer to the S_(1R) level of the red light-emittinglayer, as illustrated in FIG. 10A. However, energy is hardly transferredfrom the S_(1G) level of the green light-emitting layer to the S_(1B)level of the blue light-emitting layer.

A distance that energy can be transferred is restricted. This inhibitsemission of lights of three colors. For example, when excitons aregenerated in the blue light-emitting layer, energy is transferred fromthe singlet excited state (S_(1B)) of the blue light-emitting layer tothe singlet excited state (S_(1G)) of the green light-emitting layer, asillustrated in FIG. 10B. However, the energy is hardly transferred fromthe singlet excited state (S_(1B)) of the blue light-emitting layer tothe singlet excited state (S_(1R)) of the red light-emitting layer.

Therefore, when the red, green, and blue light-emitting layers aresimply layered, a layer in which light emission hardly occurs isproduced regardless of the layering order.

A general light-emitting layer is formed from a two-component systemincluding a host material for transporting holes and electrons and adopant (guest) material for undergoing light emission. The dopantmaterial is uniformly dispersed in the host material that is a maincomponent.

In a process of generating excitons of a light-emitting element in whicha light-emitting material is used as a dopant material, the probabilityof generating singlet excitons that are excitons in the singlet excitedstate is usually only 25%. At the remaining 75%, triplet excitons thatare excitons in the triplet excited state are generated.

A transition from the singlet excited state to the ground state is atransition between states with the same spin multiplicity. However, atransition from the triplet excited state to the ground state is atransition between states with different spin multiplicities.

The transition from the triplet excited state to the ground state is aforbidden transition, and requires a long time. Therefore, the tripletexcitons are not deactivated as light emission. The triplet excitons areconverted into thermal energy or the like, and dissipated as heat. Thus,the triplet excitons do not contribute to light emission.

A general fluorescent material (fluorescence-emitting material) thatemits fluorescence has many advantages such as excellent high-currentdensity properties and selection diversity of material. However, thesinglet excitons, of which the probability is 25%, can be only used inlight emission.

Therefore, the development of a light-emitting element using aphosphorescent material (phosphorescent compound) that emitsphosphorescence has been advanced in recent years (for example, see PTL1).

FIGS. 11A and 11B are each a view illustrating the principle of lightemission of the light-emitting element described in PTL 1.

PTL 1 discloses the light-emitting element that includes a firstelectrode, a first light-emitting layer over the first electrode,containing a first phosphorescent material and a first host material, asecond light-emitting layer over the first light-emitting layer,containing a second phosphorescent material and a second host material,a third light-emitting layer over the second light-emitting layer,containing a third phosphorescent material and a third host material,and a second electrode over the third light-emitting layer. Among peaksof emission spectra of the first, second, and third phosphorescentmaterials, the peak of the emission spectrum of the phosphorescentmaterial in the second light-emitting layer is on the longest wavelengthside, and the peak of the emission spectrum of the phosphorescentmaterial in the third light-emitting layer is on the shortest wavelengthside. The third host material has higher triplet excitation energy thanthose of the first host material and the second host material.

According to PTL 1, as illustrated in FIG. 11A, the first light-emittinglayer is a green light-emitting layer that exhibits green light emission(phosphorescence G), the second light-emitting layer is a redlight-emitting layer that exhibits red light emission (phosphorescenceR), the third light-emitting layer is a blue light-emitting layer thatexhibits blue light emission (phosphorescence B), and the ratio ofenergy transfer from a T_(1B) level of a phosphorescent material thatemits blue light to a T_(1R) level of a phosphorescent material thatemits red light and a T_(1G) level of a phosphorescent material thatemits green light is controlled by utilizing a Förster transition(resonance transition), to emit lights of three colors with balance.

In PTL 1, the phosphorescent materials are dispersed in the hostmaterial, and the mixing ratios of the phosphorescent materials aredecreased. Thus, the phosphorescent materials are isolated from eachother by the host material. This makes it difficult to cause a Dextertransition (electron exchange interaction). Therefore, the energytransfer in PTL 1 is caused mainly by the Förster transition.

The Förster transition except for a transition from the triplet excitedstate to the triplet excited state is basically a spin forbiddentransition. However, when a heavy element is contained like thephosphorescent material, spin reversal is caused by a spin orbitinteraction as illustrated in FIG. 11B. Therefore, a transition from thesinglet excited state to the singlet excited state is also allowed.

However, the phosphorescent material that emits blue light has problemsin terms of color purity and lifetime. Further, since a heavy elementsuch as iridium (Ir) is rare metal, there is a problem such as highcost. When excitons are generated in a fluorescent material, 75% of theexcitons are transferred to the T₁ level that is non-emitting. When allthese excitons are not subjected to energy transfer to anotherlight-emitting material, the excitons are dissipated as thermal energy,and the efficiency is decreased.

The disclosure has been made in view of the problems described above. Anobject of the disclosure is to provide a three-color light-emittingelement that allows each of light-emitting layers that are layered andhave different emission peak wavelengths to efficiently emit light atlow cost, a method for manufacturing the same, and a light emissionmethod.

Solution to Problem

To solve the above-described problems, a light-emitting elementaccording to one aspect of the disclosure is a light-emitting elementincluding a first electrode, a second electrode, and a functional layercontaining at least a first light-emitting layer, a secondlight-emitting layer, and a third light-emitting layer, the functionallayer being disposed between the first and second electrodes. The firstlight-emitting layer has the shortest emission peak wavelength of thelight-emitting layers and contains a host material and a TTF materialthat is a delayed fluorescent material that causes a TTF phenomenon incooperation with the host material or by the TTF material alone, orcontain at least the TTF material. The second light-emitting layer islayered on the first light-emitting layer between the firstlight-emitting layer and the third light-emitting layer. The secondlight-emitting layer contains at least a thermally activated delayedfluorescent material. The third light-emitting layer has the longestemission peak wavelength of the light-emitting layers and contains atleast a fluorescent material. The excited triplet level of at least oneof the host material and the TTF material contained in the firstlight-emitting layer is lower than the excited triplet level of thethermally activated delayed fluorescent material contained in the secondlight-emitting layer.

To solve the above-described problems, a method for manufacturing alight-emitting element according to one aspect of the disclosureincludes forming a functional layer containing at least a light-emittinglayer between first and second electrodes. The forming the functionallayer includes forming a first light-emitting layer, forming a secondlight-emitting layer, and forming a third light-emitting layer. Thefirst light-emitting layer has the shortest emission peak wavelength ofthe light-emitting layers and contains a host material and a TTFmaterial that is a delayed fluorescent material that causes a TTFphenomenon in cooperation with the host material or by the TTF materialalone, or at least the TTF material. The second light-emitting layercontains at least a thermally activated delayed fluorescent material.The excited triplet level of the thermally activated delayed fluorescentmaterial is higher than the excited triplet level of at least one of thehost material and the TTF material contained in the first light-emittinglayer. The third light-emitting layer has the longest emission peakwavelength of the light-emitting layers and contains at least afluorescent material. The formation of the first light-emitting layerand the formation of the second light-emitting layer are continuouslyperformed such that the second light-emitting layer is layered betweenthe first light-emitting layer and the third light-emitting layer andthe first light-emitting layer and the second light-emitting layer areadjacent to each other.

To solve the above-described problems, a light emission method accordingto one aspect of the disclosure is a method including transferring theenergy of excitons generated in a second light-emitting layer containingat least a thermally activated delayed fluorescent material to a firstlight-emitting layer by Dexter energy transfer, transferring the energyof excitons generated in the second light-emitting layer to a thirdlight-emitting layer by Förster energy transfer to make the first,second, and third light-emitting layers to emit light. The firstlight-emitting layer is layered on the second light-emitting layer, hasa shorter emission peak wavelength than that of the secondlight-emitting layer, and contains a host material and a TTF materialthat is a delayed fluorescent material that causes a TTF phenomenon incooperation with the host material or by the TTF material alone, or atleast the TTF material. The excited triplet level of at least one of thehost material and the TTF material is lower than the excited tripletlevel of the thermally activated delayed fluorescent material. The thirdlight-emitting layer is layered on the second light-emitting layer on aside opposite to the first light-emitting layer, has a longer emissionpeak wavelength than that of the second light-emitting layer, andcontains a host material or a fluorescent material, or at least thefluorescent material.

Advantageous Effects of Invention

According to an aspect of the disclosure, a three-color light-emittingelement that allows each of light-emitting layers that are layered andhave different emission peak wavelengths to efficiently emit light atlow cost, a method for manufacturing the same, and a light emissionmethod can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are views illustrating the principle of light emissionof a light-emitting element according to a first embodiment of thedisclosure.

FIG. 2 is a cross-sectional view of an example of a schematicconfiguration of the light-emitting element according to the firstembodiment of the disclosure.

FIG. 3 is a cross-sectional view of an example of a schematicconfiguration of a light-emitting element according to a secondembodiment of the disclosure.

FIG. 4 is a cross-sectional view of an example of a schematicconfiguration of a light-emitting element according to a thirdembodiment of the disclosure and a main portion of an electronic deviceprovided with the light-emitting element.

FIG. 5 is another cross-sectional view of the example of the schematicconfiguration of the light-emitting element according to the thirdembodiment of the disclosure and the main portion of the electronicdevice provided with the light-emitting element.

FIG. 6 is a cross-sectional view of an example of a schematicconfiguration of a light-emitting element according to a fourthembodiment of the disclosure.

FIG. 7 is a cross-sectional view of an example of a schematicconfiguration of a light-emitting element according to a fifthembodiment of the disclosure.

FIG. 8 are a view illustrating an energy diagram of each of layersbetween which a first light-emitting layer is provided in thelight-emitting element according to the fifth embodiment of thedisclosure.

FIG. 9 is a cross-sectional view of an example of a schematicconfiguration of a light-emitting element according to a sixthembodiment of the disclosure.

FIGS. 10A and 10B are each a view illustrating a problem of knownlight-emitting element in which red, green, and blue light-emittinglayers are simply layered.

FIGS. 11A and 11B are each a view illustrating the principle of lightemission of the light-emitting element described in PTL 1.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the disclosure will be described in detail.

First Embodiment

One embodiment of the disclosure will be described hereinafter withreference to FIGS. 1A and 1B and 2.

FIGS. 1A and 1B are views illustrating the principle of light emissionof a light-emitting element according to the present embodiment. FIG. 2is a cross-sectional view of an example of a schematic configuration ofthe light-emitting element according to the present embodiment.

In the light-emitting element according to the present embodiment, afunctional layer containing at least a light-emitting layer is providedbetween first and second electrodes. The functional layer may be anorganic layer or an inorganic layer. Hereinafter, an organic EL elementwill be described as an example of the light-emitting element accordingto the present embodiment.

Schematic Configuration of Organic EL Element

As illustrated in FIG. 2, an organic EL element 10 according to thepresent embodiment has a configuration in which a positive electrode 2(first electrode), an organic EL layer 3 (organic layer or functionallayer), and a negative electrode 4 (second electrode) are layered on asubstrate 1 in this order from the substrate 1 side.

The organic EL layer 3 is a light-emitting unit including an organiclayer containing a light-emitting layer 33. The organic EL element 10according to the present embodiment is a light-emitting device forillumination capable of white (W) display. The organic EL element 10 isa top-emitting organic EL element. In the organic EL element 10, lightemitted from the light-emitting layer 33 is extracted as white lightfrom a side opposite to the substrate 1.

Hereinafter, each of the components described above will be described indetail.

Substrate 1

The substrate 1 is not particularly limited as long as it has aninsulating property. For example, a publicly known insulating substratemay be used.

As the substrate 1, for example, an inorganic substrate formed fromglass or quartz or a plastic substrate formed from polyethyleneterephthalate or a polyimide resin may be used.

In the present embodiment, a case where a glass substrate (transparentsubstrate) is used as an insulating substrate having alight-transmitting property will be described later as an example of thesubstrate 1. However, in the top-emitting organic EL element 10, thesubstrate 1 does not need a light-transmitting property as describedabove.

Therefore, when the organic EL element 10 is a top-emitting organic ELelement, a semiconductor substrate such as a silicon wafer, a substratein which a surface of a metal substrate formed from aluminum (Al) oriron (Fe) is coated with an insulating material such as silicon oxideand an organic insulating material, a substrate in which a surface of ametal substrate formed from Al is subjected to an insulation treatmentby an anodic oxidation method, or the like may be used as the substrate1.

On the substrate 1, a circuit including a drive element such as a TFTmay be formed.

Positive Electrode 2 and Negative Electrode 4

The positive electrode 2 and the negative electrode 4 are a pair ofelectrodes. The positive electrode 2 functions as an electrode forinjecting (supplying) holes (h⁺) into the organic EL layer 3. On theother hand, the negative electrode 4 functions as an electrode forinjecting (supplying) electrons (e⁻) into the organic EL layer 3.

The shape, structure, and size of the positive electrode 2 and thenegative electrode 4 are not particularly limited, and can beappropriately selected according to the application and purpose of theorganic EL element 10.

Electrode materials capable of being employed for the positive electrode2 and the negative electrode 4 are not particularly limited to aspecific material, and, for example, a publicly known electrode materialmay be employed therefor.

For the positive electrode 2, for example, a metal such as gold (Au),platinum (Pt), and nickel (Ni), or a transparent electrode material suchas indium tin oxide (ITO), tin oxide (SnO₂), indium tin oxide (IZO), andgallium doped zinc oxide (GZO) may be used.

On the other hand, it is preferable that a material having a small workfunction be used for the negative electrode 4 to inject electrons intothe light-emitting layer 33. For the negative electrode 4, for example,a metal such as lithium (Li), calcium (Ca), cerium (Ce), barium (Ba),and aluminum (Al) or an alloy containing the metal, such as an Ag—Mgalloy and an Al—Li alloy may be used.

The thicknesses of the positive electrode 2 and the negative electrode 4are not limited to a specific thickness, and may be set similar to thosein a known EL element.

Light generated in the light-emitting layer 33 needs to be extractedfrom at least one of the positive electrode 2 and the negative electrode4. In general, light is extracted from any one electrode side of thepositive electrode 2 and the negative electrode 4. It is preferable thatfor an electrode in which light is extracted, a light-transmittingelectrode material that transmits light be used, and for an electrode inwhich light is not extracted, a non-translucent electrode material thatdoes not transmit light be used.

That is, various electrically conductive materials may be used for thepositive electrode 2 and the negative electrode 4. However when theorganic EL element 10 is a top-emitting EL element, it is preferablethat an electrode on a side of the substrate 1 (in the presentembodiment, the positive electrode 2) be formed from a non-translucentelectrode material and an electrode on a side opposite to the substrate1 with the organic EL layer 3 interposed between the electrode and thesubstrate 1 (in the present embodiment, the negative electrode 4) beformed from a transparent light-transmitting electrode material(transparent electrode) or a translucent light-transmitting electrodematerial (translucent electrode).

The positive electrode 2 and the negative electrode 4 may each have asingle layer formed from one electrode material or a layered structureformed from a plurality of electrode materials.

Thus, when the organic EL element 10 is a top-emitting EL element asdescribed above, the positive electrode 2 may have a layered structureincluding a non-translucent electrode 21 formed from a non-translucentelectrode material, such as a reflective electrode, and alight-transmitting electrode 22 formed from a light-transmittingelectrode material, as illustrated in FIG. 2.

Examples of the non-translucent electrode material include a blackelectrode material such as tantalum (Ta) and carbon (C), and areflective metal electrode material such as Al, silver (Ag), gold (Au),an Al—Li alloy, an Al-neodymium (Nd) alloy, and an Al-silicon (Si)alloy.

As the light-transmitting electrode material, for example, a transparentelectrode material described above may be employed, or a translucentelectrode material such as a thin film of Ag may be used.

Organic EL Layer 3

As illustrated in FIG. 2, the organic EL layer 3 according to thepresent embodiment has a configuration in which a hole injecting layer31, a hole transport layer 32, the light-emitting layer 33, an electrontransport layer 34, and an electron injecting layer 35 are layered inthis order from the positive electrode 2 side.

An organic EL layer other than the light-emitting layer 33 is not alayer necessary as the organic EL layer 3, and may be appropriatelyformed according to required properties of the organic EL element 10.

Light-Emitting Layer 33

The light-emitting layer 33 has a three-layer structure including afirst light-emitting layer 33 a (first light-emitting layer), a secondlight-emitting layer 33 b (second light-emitting layer), and a thirdlight-emitting layer 33 c (third light-emitting layer).

The second light-emitting layer 33 b is layered on the firstlight-emitting layer 33 a between the first light-emitting layer 33 aand the third light-emitting layer 33 c. The organic EL element 10according to the present embodiment has a simple structure in which alayer other than the light-emitting layers (intermediate layer) is notprovided between the first light-emitting layer 33 a, the secondlight-emitting layer 33 b, and the third light-emitting layer 33 c.Therefore, in the present embodiment, the second light-emitting layer 33b is disposed adjacent to both the first light-emitting layer 33 a andthe third light-emitting layer 33 c between the first light-emittinglayer 33 a and the third light-emitting layer 33 c. As described in anembodiment described later, an intermediate layer that does not containan illuminant and has a higher S₁ level than the S₁ level of the thirdlight-emitting layer 33 c may be provided between the secondlight-emitting layer 33 b and the third light-emitting layer 33 c, andthe second light-emitting layer 33 b and the third light-emitting layer33 c do not need to be disposed adjacent to each other. In the presentembodiment, the first light-emitting layer 33 a, the secondlight-emitting layer 33 b, and the third light-emitting layer 33 c arelayered in this order from the positive electrode 2 side.

Of the light-emitting layer 33, the first light-emitting layer 33 aemits light having the shortest emission peak wavelength, the thirdlight-emitting layer 33 c emits light having the longest emission peakwavelength, and the second light-emitting layer 33 b emits light havingan emission peak wavelength that is from the emission peak wavelength oflight in the first light-emitting layer 33 a to the emission peakwavelength of light in the third light-emitting layer 33 c.

In the present embodiment, as a light-emitting material, aphosphorescent material is not used, but a delayed fluorescent materialis used for each of the first light-emitting layer 33 a and the secondlight-emitting layer 33 b. A fluorescent material containing at leasttwo kinds of delayed fluorescent material (fluorescence-emittingmaterial and phosphor) is used to perform white light emission bythree-color light emission.

Therefore, in the present embodiment, for the first light-emitting layer33 a, a delayed fluorescent material that emits light having theshortest emission peak wavelength of the light-emitting materials usedfor the light-emitting layer 33 is used. For the third light-emittinglayer 33 c, a fluorescent material that emits light having the longestemission peak wavelength of the light-emitting material used for thelight-emitting layer 33 is used. For the second light-emitting layer 33b, a delayed fluorescent material that emits light having an emissionpeak wavelength that is from the shortest emission peak wavelength tothe longest emission peak wavelength is used.

The first light-emitting layer 33 a, the second light-emitting layer 33b, and the third light-emitting layer 33 c may be each formed from ahost material and the fluorescent material, that is, a dopant materialexhibiting fluorescence (light-emitting dopant and guest material), orfrom the dopant material alone. The host material is a compound in whichholes and electrons can be injected, and has a function of allowing alight-emitting dopant to emit light by holes and electrons that aretransported and recombine with each other within a molecule thereof.

Hereinafter, in the present embodiment, a case where the firstlight-emitting layer 33 a is a blue light-emitting layer (B fluorescentlayer), the second light-emitting layer 33 b is a green light-emittinglayer (G fluorescent layer), and the third light-emitting layer 33 c isa red light-emitting layer (R fluorescent layer) will be described as anexample.

For the blue light-emitting layer, for example, a fluorescent materialhaving a peak wavelength within a wavelength range from 380 nm to 480 nmis used. For the green light-emitting layer, for example, a fluorescentmaterial having a peak wavelength within a wavelength range from 510 nmto 560 nm is used. For the red light-emitting layer, for example, afluorescent material having a peak wavelength within a wavelength rangefrom 600 nm to 680 nm is used.

First Light-Emitting Layer 33 a

In the present embodiment, as the delayed fluorescent materialsdescribed above, a Triplet-Triplet-Fusion (TTF) material and a ThermallyActivated Delayed Fluorescent (TADF) material are used.

For the first light-emitting layer 33 a, the TTF material is used as thedelayed fluorescent material described above. The first light-emittinglayer 33 a contains the host material (first host material) and the TTFmaterial or at least the TTF material.

The TTF material is a delayed fluorescent material in which lightemission can be achieved by re-excitation from the T₁ level to the S₁level in cooperation with the host material or by the TTF materialalone. In the delayed fluorescent material, a triplet-triplet phenomenon(also referred to as Triplet-Triplet Annihilation (TTA)) (hereinaftersometimes simply referred to as TTF) in which a singlet exciton isgenerated from a plurality of triplet excitons by collision and fusionof triplet excitons is caused, and as a result, light is emitted. It isconsidered that use of delayed fluorescence by the TTF phenomenon cantheoretically enhance the internal quantum efficiency to 40% influorescent emission.

In the present embodiment, the TTF material in which upconversion fromthe T₁ level of the fluorescent material or the T₁ level of the hostmaterial to the S₁ level of the fluorescent material by TTF occurs toemit delayed fluorescence is used for the first light-emitting layer 33a.

When the first light-emitting layer 33 a contains the host material andthe dopant material including the fluorescent material, it is preferablethat the triplet energy E_(Th) of the host material be lower than thetriplet energy E_(Td) of the dopant material. When this relationship issatisfied, triplet excitons generated on the host material are nottransferred to the dopant material that has a higher triplet energy, andtriplet excitons generated on the dopant material are rapidlytransferred to the host material as energy. Therefore, the tripletexcitons of the host material are not transferred to the dopantmaterial, and the triplet excitons efficiently collide with each otheron the host material by the TTF phenomenon, to generate singletexcitons. Further, it is preferable that the singlet energy E_(Sd) ofthe dopant material be lower than the singlet energy E_(Sh) of the hostmaterial. When this relationship is satisfied, singlet excitonsgenerated by the TTF phenomenon are transferred from the host materialto the dopant material as energy. The singlet excitons contribute tofluorescent emission of the dopant material.

Herein, the triplet energy represents the lowest triplet state (T₁),that is, in the present embodiment, a difference between energy inT_(1B) and energy in the ground state (S₀) in FIG. 1A. The singletenergy (also referred to as energy gap) represents the lowest singletstate (S₁), that is, in the present embodiment, a difference betweenenergy in S_(1B) and the energy in the ground state (S₀) in FIG. 1A.

In a dopant material used for a fluorescent element, a transition fromthe triplet excited state to the ground state is originally a forbiddentransition. In such a transition, the triplet excitons are notdeactivated as light emission. The triplet excitons are converted intothermal energy and dissipated as heat. Thus, the triplet excitons do notcontribute to light emission.

However, when the host material and the dopant material satisfy therelationships described above, the triplet excitons collide with eachother to efficiently generate the singlet excitons, before thermaldeactivation, and the light-emitting efficiency is improved.

It is desirable that the affinity Ah of the host material be higher thanthe affinity Ad of the dopant material and the ionization potential Ihof the host material be higher than the ionization potential of thedopant material.

The affinity means energy to be released or absorbed when an electron isgiven to the molecule of the host material. In a case of release, theaffinity is defined as positive, and in a case of absorption, theaffinity is defined as negative.

The affinity of the host material or the dopant material is representedby Af=Ig−Eg, wherein Ip (Ih or Id) is an ionization potential of thehost material or the dopant material, and Eg (E_(Sh) or E_(Sd)) is asinglet energy thereof.

The dopant material used for the first light-emitting layer 33 a is afluorescent material having the shortest emission peak wavelength asdescribed above. Specifically, a blue light-emitting phosphor thatgenerates fluorescence having a peak wavelength within a wavelengthrange from 380 nm to 480 nm. The dopant material has a relatively largeenergy gap. Therefore, when the dopant material used for the firstlight-emitting layer 33 a satisfies that Ah is higher than Ad, thedopant material also satisfies that Ih is higher than Id.

When a difference in ionization potential between the host material andthe dopant material is larger, the dopant material has a hole-trappingproperty. In this case, the triplet excitons are generated not only on ahost molecule (the host material) but also directly on a dopant molecule(the dopant material). When the relationship of E_(Th)<E_(Td) issatisfied as described, the triplet exciton energy on the dopantmolecules is transferred onto the host molecule by Dexter energytransfer (Dexter transition) due to an electron exchange interaction(i.e., Dexter mechanism). All the triplet excitons are collected on thehost molecule. As a result, the TTF phenomenon occurs with efficiency.

For example, a combination of the host material and the dopant material(TTF material) that cause the TTF phenomenon can be selected from thefollowing compounds.

Examples of the host material include an anthracene derivative and apolycyclic aromatic skeleton-containing compound. The host material maybe used alone, or two or more kinds thereof may be appropriately used incombination.

Examples of the dopant material (TTF material) include a pyrenederivative such as an aminopyrene derivative, an aminoanthracenederivative, and aminochrysene derivative. The dopant material may beused alone, or two or more kinds thereof may be appropriately used incombination.

In particular, it is preferable that an anthracene derivative and atleast one selected from an aminoanthracene derivative, an aminochrysenederivative, and an aminopyrene derivative be used as the host materialand the dopant material, respectively, in combination. It is morepreferable that an anthracene derivative and at least one selected froman aminochrysene derivative and an aminopyrene derivative be used as thehost material and the dopant material, respectively, in combination.

Examples of the anthracene derivative used as the host material includea compound represented by Formula (1) below.

Such a compound is preferable.

In Formula (1), Ar¹¹ and Ar¹² are each independently a substituted orunsubstituted aryl group having from 6 to 50 ring-forming carbon atomsor a heterocyclic group having from 5 to 50 ring-forming atoms. R¹ to R⁸are each independently a group selected from a hydrogen atom, asubstituted or unsubstituted aryl group having from 6 to 50 ring-formingcarbon atoms, a substituted or unsubstituted heterocyclic group havingfrom 5 to 50 ring-forming atoms, a substituted or unsubstituted alkylgroup having from 1 to 50 carbon atoms, a substituted or unsubstitutedcycloalkyl group having from 3 to 50 ring-forming carbon atoms, asubstituted or unsubstituted alkoxy group having from 1 to 50 carbonatoms, a substituted or unsubstituted aralkyl group having from 7 to 50carbon atoms, a substituted or unsubstituted aryloxy group having from 6to 50 ring-forming carbon atoms, a substituted or unsubstituted arylthiogroup having from 6 to 50 ring-forming carbon atoms, a substituted orunsubstituted alkoxycarbonyl group having from 2 to 50 carbon atoms, asubstituted or unsubstituted silyl group, a carboxyl group, a halogenatom, a cyano group, a nitro group and a hydroxyl group.

The anthracene derivative is not particularly limited, and may beappropriately selected according to the configuration and requiredproperties of the used organic EL element 10.

Examples of the anthracene derivative used in the present embodimentinclude an anthracene derivative represented by Formula (1), whereinAr¹¹ and Ar¹² are each independently a substituted or unsubstitutedfused aryl group having from 10 to 50 ring-forming carbon atoms.

Such an anthracene derivative can include an anthracene derivative inwhich Ar¹¹ and Ar¹² are the same substituted or unsubstituted fused arylgroup and an anthracene derivative in which Ar¹¹ and Ar¹² are differentsubstituted or unsubstituted fused aryl groups.

Examples of the anthracene derivative in which Ar¹¹ and Ar¹² are thesame substituted or unsubstituted fused aryl group include anthracenederivatives represented by Formulae (2) to (4) below.

Such derivatives are used as the anthracene derivative.

In the anthracene derivative represented by Formula (2), Ar¹¹ and Ar¹²are a substituted or unsubstituted 9-phenanthrenyl group. In Formula(2), R¹ to R⁸ are as defined above. R¹¹ is a group selected from ahydrogen atom, a substituted or unsubstituted aryl group having from 6to 50 ring-forming carbon atoms, a substituted or unsubstitutedheterocyclic group having from 5 to 50 ring-forming atoms, a substitutedor unsubstituted alkyl group having from 1 to 50 carbon atoms, asubstituted or unsubstituted cycloalkyl group having from 3 to 50ring-forming carbon atoms, a substituted or unsubstituted alkoxy grouphaving from 1 to 50 carbon atoms, a substituted or unsubstituted aralkylgroup having from 7 to 50 carbon atoms, a substituted or unsubstitutedaryloxy group having from 6 to 50 ring-forming carbon atoms, asubstituted or unsubstituted arylthio group having from 6 to 50ring-forming carbon atoms, a substituted or unsubstituted alkoxycarbonylgroup having from 2 to 50 carbon atoms, a substituted or unsubstitutedsilyl group, a carboxyl group, a halogen atom, a cyano group, a nitrogroup, and a hydroxyl group. a is an integer from 0 to 9. When a is aninteger of not less than 2, R¹¹s may be the same as or different fromeach other as long as two substituted or unsubstituted phenanthrenylgroups are the same.

In the anthracene derivative represented by Formula (3), Ar¹¹ and Ar¹²in Formula (1) are a substituted or unsubstituted 2-naphthyl group. InFormula (3), R¹ to R⁸ and R¹¹ are as defined above. b is an integer from1 to 7. When b is an integer of not less than 2, R¹¹s may be the same asor different from each other as long as two substituted or unsubstituted2-naphthyl groups are the same.

In the anthracene derivative represented by Formula (4), Ar¹¹ and Ar¹²in Formula (1) are a substituted or unsubstituted 1-naphthyl group. InFormula (4), R¹ to R⁸, R¹¹, and b are as defined above. When b is aninteger of not less than 2, R¹¹s may be the same as or different fromeach other as long as two substituted or unsubstituted 1-naphthyl groupsare the same.

Examples of the anthracene derivative represented by Formula (1),wherein Ar¹¹ and Ar¹² are different substituted or unsubstituted fusedaryl groups include anthracene derivatives in which Ar¹¹ and Ar¹² are asubstituted or unsubstituted 9-phenanthrenyl group, a substituted orunsubstituted 1-naphthyl group, or a substituted or unsubstituted2-naphthyl group.

Specific examples thereof include an anthracene derivative in which Ar¹¹is a 1-naphthyl group and Ar¹² is a 2-naphthyl group, an anthracenederivative in which Ar¹¹ is a 1-naphthyl group and Ar¹² is a9-phenanthryl group, and an anthracene derivative in which Ar¹¹ is a2-naphthyl group and Ar¹² is a 9-phenanthryl group.

The anthracene derivative used in the present embodiment may be ananthracene derivative represented by Formula (1), wherein one of Ar¹¹and Ar¹² is a substituted or unsubstituted phenyl group and the other isa substituted or unsubstituted fused aryl group having from 10 to 50fused aryl groups.

Examples of such an anthracene derivative include anthracene derivativesrepresented by Formulae (5) and (6) below.

Such derivatives are used as the anthracene derivative.

In the anthracene derivative represented by Formula (5), Ar¹¹ in Formula(1) is a substituted or unsubstituted 1-naphthyl group and Ar¹² inFormula (1) is a substituted or unsubstituted phenyl group. In Formula(5), R¹ to R⁸, R¹¹, and b are as defined above. Ar⁶ is a substituted orunsubstituted aryl group having from 6 to 50 ring-forming carbon atoms,a substituted or unsubstituted alkyl group having from 1 to 50 carbonatoms, a substituted or unsubstituted cycloalkyl group having from 3 to50 ring-forming carbon atoms, a substituted or unsubstituted aralkylgroup having from 7 to 50 carbon atoms, a substituted or unsubstitutedheterocyclic group having from 5 to 50 ring-forming atoms, a9,9-dimethylfluoren-1-yl group, a 9,9-dimethylfluoren-2-yl group, a9,9-dimethylfluoren-3-yl group, a 9,9-dimethylfluoren-4-yl group, adibenzofuran-1-yl group, a dibenzofuran-2-yl group, a dibenzofuran-3-ylgroup, or a dibenzofuran-4-yl group. Ar⁶ may form a ring, such as asubstituted or unsubstituted fluorenyl group, or a substituted orunsubstituted dibenzofuranyl group, with a benzene ring to which Ar⁶ isbonded. When b is an integer of not less than 2, R¹¹s may be the same asor different from each other.

In the anthracene derivative represented by Formula (6), Ar¹¹ in Formula(1) is a substituted or unsubstituted 2-naphthyl group and Ar¹² is asubstituted or unsubstituted phenyl group. In Formula (6), R¹ to R⁸,R¹¹, and b are as defined above. Ar⁷ is a substituted or unsubstitutedaryl group having from 6 to 50 ring-forming carbon atoms, a substitutedor unsubstituted heterocyclic group having from 5 to 50 ring-formingatoms, a substituted or unsubstituted alkyl group having from 1 to 50carbon atoms, a substituted or unsubstituted cycloalkyl group havingfrom 3 to 50 ring-forming carbon atoms, a substituted or unsubstitutedaralkyl group having from 7 to 50 carbon atoms, a dibenzofuran-1-ylgroup, a dibenzofuran-2-yl group, a dibenzofuran-3-yl group, or adibenzofuran-4-yl group. Ar⁷ may form a ring, such as a substituted orunsubstituted fluorenyl group, or a substituted or unsubstituteddibenzofuranyl group, with a benzene ring to which Ar⁷ is bonded. When bis an integer of not less than 2, R¹¹s may be the same as or differentfrom each other.

The anthracene derivative used in the present embodiment may be, forexample, an anthracene derivative represented by Formula (7) below.

Such an anthracene derivative may be used in the present embodiment.

In Formula (7), R¹ to R⁸ and Ar⁶ are as defined above. Ar⁵ is asubstituted or unsubstituted aryl group having from 6 to 50 ring-formingcarbon atoms, a substituted or unsubstituted alkyl group having from 1to 50 carbon atoms, a substituted or unsubstituted cycloalkyl grouphaving from 3 to 50 ring-forming carbon atoms, a substituted orunsubstituted aralkyl group having from 7 to 50 carbon atoms, or asubstituted or unsubstituted heterocyclic group having from 5 to 50ring-forming atoms. Ar⁵ and Ar⁶ are each independently selected.

Examples of such an anthracene derivative include anthracene derivativesrepresented by Formulae (8) to (10) below.

Such anthracene derivatives are used in the present embodiment.

In Formulae (8) to (10), R¹ to R⁸ are as defined above.

In Formula (9), Ar⁸ is a substituted or unsubstituted fused aryl grouphaving from 10 to 20 ring-forming carbon atoms.

In Formula (10), Ar^(5a) and Ar^(6a) are each independently asubstituted or unsubstituted fused aryl group having from 10 to 20ring-forming carbon atoms.

Examples of the substituted or unsubstituted aryl groups having from 6to 50 ring-forming carbon atoms of R¹ to R⁸, R¹¹, Ar⁵ to Ar⁷, Ar¹¹, andAr¹² in Formulae (1) to (10) include a phenyl group, a 1-naphthyl group,a 2-naphthyl group, a 1-anthryl group, a 2-anthryl group, a 9-anthrylgroup, a 1-phenanthryl group, a 2-phenanthryl group, a 3-phenanthrylgroup, a 4-phenanthryl group, a 9-phenanthryl group, a 1-naphthacenylgroup, a 2-naphthacenyl group, a 9-naphthacenyl group, a 1-pyrenylgroup, a 2-pyrenyl group, a 4-pyrenyl group, a 6-chrysenyl group, a1-benzo[c]phenanthryl group, a 2-benzo[c]phenanthryl group, a3-benzo[c]phenanthryl group, a 4-benzo[c]phenanthryl group, a5-benzo[c]phenanthryl group, a 6-benzo[c]phenanthryl group, a1-benzo[g]chrysenyl group, a 2-benzo[g]chrysenyl group, a3-benzo[g]chrysenyl group, a 4-benzo[g]chrysenyl group, a5-benzo[g]chrysenyl group, a 6-benzo[g]chrysenyl group, a7-benzo[g]chrysenyl group, a 8-benzo[g]chrysenyl group, a9-benzo[g]chrysenyl group, a 10-benzo[g]chrysenyl group, a11-benzo[g]chrysenyl group, a 12-benzo[g]chrysenyl group, a13-benzo[g]chrysenyl group, a 14-benzo[g]chrysenyl group, a 1-triphenylgroup, a 2-triphenyl group, a 2-fluorenyl group, a9,9-dimethylfluoren-2-yl group, a benzofluorenyl group, adibenzofluorenyl group, a 2-biphenylyl group, a 3-biphenylyl group, a4-biphenylyl group, a p-terphenyl-4-yl group, a p-terphenyl-3-yl group,a p-terphenyl-2-yl group, a m-terphenyl-4-yl group, a m-terphenyl-3-ylgroup, a m-terphenyl-2-yl group, an o-tolyl group, an m-tolyl group, ap-tolyl group, a p-t-butylphenyl group, a p-(2-phenylpropyl)phenylgroup, a 3-methyl-2-naphthyl group, a 4-methyl-1-naphthyl group, a4-methyl-1-anthryl group, a 4′-methylbiphenyl group, and a4″-t-butyl-p-terphenyl-4-yl group. Among these, an unsubstituted phenylgroup, a substituted phenyl group, a substituted or unsubstituted arylgroup having from 10 to 14 ring-forming carbon atoms (e.g., a 1-naphthylgroup, a 2-naphthyl group, and a 9-phenanthryl group), a substituted orunsubstituted fluorenyl group (a 2-fluorenyl), and a substituted orunsubstituted pyrenyl group (a 1-pyrenyl group, 2-pyrenyl group, and4-pyrenyl group) are preferable.

Examples of the substituted or unsubstituted fused aryl groups havingfrom 10 to 20 ring-forming carbon atoms of Ar^(5a), Ar^(6a), and Ar⁸include a 1-naphthyl group, a 2-naphthyl group, a 1-anthryl group, a2-anthryl group, a 9-anthryl group, a 1-phenanthryl group, a2-phenanthryl group, a 3-phenanthryl group, a 4-phenanthryl group, a9-phenanthryl group, a 1-naphthacenyl group, a 2-naphthacenyl group, a9-naphthacenyl group, a 1-pyrenyl group, a 2-pyrenyl group, a 4-pyrenylgroup, and a 2-fluorenyl group. Among these, a 1-naphthyl group, a2-naphthyl group, a 9-phenanthryl group, and a fluorenyl group (a2-fluorenyl group) are preferable.

Examples of the substituted or unsubstituted heterocyclic groups havingfrom 5 to 50 ring-forming atoms of R¹ to R⁸, R¹¹, Ar⁵ to Ar⁷, Ar¹¹, andAr¹² include a 1-pyrrolyl group, a 2-pyrrolyl group, a 3-pyrrolyl group,a pyrazinyl group, a 2-pyridinyl group, a 3-pyridinyl group, a4-pyridinyl group, a 1-indolyl group, a 2-indolyl group, a 3-indolylgroup, a 4-indolyl group, a 5-indolyl group, a 6-indolyl group, a7-indolyl group, a 1-isoindolyl group, a 2-isoindolyl group, a3-isoindolyl group, a 4-isoindolyl group, a 5-isoindolyl group, a6-isoindolyl group, a 7-isoindolyl group, a 2-furyl group, a 3-furylgroup, a 2-benzofuranyl group, a 3-benzofuranyl group, a 4-benzofuranylgroup, a 5-benzofuranyl group, a 6-benzofuranyl group, a 7-benzofuranylgroup, a 1-isobenzofuranyl group, a 3-isobenzofuranyl group, a4-isobenzofuranyl group, a 5-isobenzofuranyl group, a 6-isobenzofuranylgroup, a 7-isobenzofuranyl group, a 1-dibenzofuranyl group, a2-dibenzofuranyl group, a 3-dibenzofuranyl group, a 4-dibenzofuranylgroup, a 1-dibenzothiophenyl group, a 2-dibenzothiophenyl group, a3-dibenzothiophenyl group, a 4-dibenzothiophenyl group, a quinolylgroup, a 3-quinolyl group, a 4-quinolyl group, a 5-quinolyl group, a6-quinolyl group, a 7-quinolyl group, a 8-quinolyl group, a1-isoquinolyl group, a 3-isoquinolyl group, a 4-isoquinolyl group, a5-isoquinolyl group, a 6-isoquinolyl group, a 7-isoquinolyl group, a8-isoquinolyl group, a 2-quinoxalinyl group, a 5-quinoxalinyl group, a6-quinoxalinyl group, a 1-carbazolyl group, a 2-carbazolyl group, a3-carbazolyl group, a 4-carbazolyl group, a 9-carbazolyl group, a1-phenanthrydinyl group, a 2-phenanthrydinyl group, a 3-phenanthrydinylgroup, a 4-phenanthrydinyl group, a 6-phenanthrydinyl group, a7-phenanthrydinyl group, a 8-phenanthrydinyl group, a 9-phenanthrydinylgroup, a 10-phenanthrydinyl group, a 1-acridinyl group, a 2-acridinylgroup, a 3-acridinyl group, a 4-acridinyl group, a 9-acridinyl group, a1,7-phenanthrolin-2-yl group, a 1,7-phenanthrolin-3-yl group, a1,7-phenanthrolin-4-yl group, a 1,7-phenanthrolin-5-yl group, a1,7-phenanthrolin-6-yl group, a 1,7-phenanthrolin-8-yl group, a1,7-phenanthrolin-9-yl group, a 1,7-phenanthrolin-10-yl group, a1,8-phenanthrolin-2-yl group, a 1,8-phenanthrolin-3-yl group, a1,8-phenanthrolin-4-yl group, a 1,8-phenanthrolin-5-yl group, a1,8-phenanthrolin-6-yl group, a 1,8-phenanthrolin-7-yl group, a1,8-phenanthrolin-9-yl group, a 1,8-phenanthrolin-10-yl group, a1,9-phenanthrolin-2-yl group, a 1,9-phenanthrolin-3-yl group, a1,9-phenanthrolin-4-yl group, a 1,9-phenanthrolin-5-yl group, a1,9-phenanthrolin-6-yl group, a 1,9-phenanthrolin-7-yl group, a1,9-phenanthrolin-8-yl group, a 1,9-phenanthrolin-10-yl group, a1,10-phenanthrolin-2-yl group, a 1,10-phenanthrolin-3-yl group, a1,10-phenanthrolin-4-yl group, a 1,10-phenanthrolin-5-yl group, a2,9-phenanthrolin-1-yl group, a 2,9-phenanthrolin-3-yl group, a2,9-phenanthrolin-4-yl group, a 2,9-phenanthrolin-5-yl group, a2,9-phenanthrolin-6-yl group, a 2,9-phenanthrolin-7-yl group, a2,9-phenanthrolin-8-yl group, a 2,9-phenanthrolin-10-yl group, a2,8-phenanthrolin-1-yl group, a 2,8-phenanthrolin-3-yl group, a2,8-phenanthrolin-4-yl group, a 2,8-phenanthrolin-5-yl group, a2,8-phenanthrolin-6-yl group, a 2,8-phenanthrolin-7-yl group, a2,8-phenanthrolin-9-yl group, a 2,8-phenanthrolin-10-yl group, a2,7-phenanthrolin-1-yl group, a 2,7-phenanthrolin-3-yl group, a2,7-phenanthrolin-4-yl group, a 2,7-phenanthrolin-5-yl group, a2,7-phenanthrolin-6-yl group, a 2,7-phenanthrolin-8-yl group, a2,7-phenanthrolin-9-yl group, a 2,7-phenanthrolin-10-yl group, a1-phenadinyl group, a 2-phenadinyl group, a 1-phenothiadinyl group, a2-phenothiadinyl group, a 3-phenothiadinyl group, a 4-phenothiadinylgroup, a 10-phenothiadinyl group, a 1-phenoxadinyl group, a2-phenoxadinyl group, a 3-phenoxadinyl group, a 4-phenoxadinyl group, a10-phenoxadinyl group, a 2-oxazolyl group, a 4-oxazolyl group, a5-oxazolyl group, a 2-oxadiazolyl group, a 5-oxadiazolyl group, a3-furazanyl group, a 2-thienyl group, a 3-thienyl group, a2-methylpyrrol-1-yl group, a 2-methylpyrrol-3-yl group, a2-methylpyrrol-4-yl group, a 2-methylpyrrol-5-yl group, a3-methylpyrrol-1-yl group, a 3-methylpyrrol-2-yl group, a3-methylpyrrol-4-yl group, a 3-methylpyrrol-5-yl group, a2-t-butylpyrrol-4-yl group, a 3-(2-phenylpropyl)pyrrol-1-yl group, a2-methyl-1-indolyl group, a 4-methyl-1-indolyl group, a2-methyl-3-indolyl group, a 4-methyl-3-indolyl group, a2-t-butyl-1-indolyl group, a 4-t-butyl-1-indolyl group, a2-t-butyl-3-indolyl group, and a 4-t-butyl-3-indolyl group. Among these,a 1-dibenzofuranyl group, a 2-dibenzofuranyl group, a 3-dibenzofuranylgroup, a 4-dibenzofuranyl group, a 1-dibenzothiophenyl group, a2-dibenzothiophenyl group, a 3-dibenzothiophenyl group, a4-dibenzothiophenyl group, a 1-carbazolyl group, a 2-carbazolyl group, a3-carbazolyl group, a 4-carbazolyl group, and a 9-carbazolyl group arepreferable.

Examples of the substituted or unsubstituted alkyl groups having from 1to 50 carbon atoms of R¹ to R⁸, R¹¹, and Ar⁵ to Ar⁷ include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a n-butylgroup, an s-butyl group, an isobutyl group, a t-butyl group, a n-pentylgroup, a n-hexyl group, a n-heptyl group, a n-octyl group, ahydroxymethyl group, a 1-hydroxyethyl group, a 2-hydroxyethyl group, a2-hydroxyisobutyl group, a 1,2-dihydroxyethyl group, a1,3-dihydroxyisopropyl group, a 2,3-dihydroxy-t-butyl group, a1,2,3-trihydroxypropyl group, a chloromethyl group, a 1-chloroethylgroup, a 2-chloroethyl group, a 2-chloroisobutyl group, a1,2-dichloroethyl group, a 1,3-dichloroisopropyl group, a2,3-dichloro-t-butyl group, a 1,2,3-trichloropropyl group, a bromomethylgroup, a 1-bromoethyl group, a 2-bromoethyl group, a 2-bromoisobutylgroup, a 1,2-dibromoethyl group, a 1,3-dibromoisopropyl group, a2,3-dibromo-t-butyl group, a 1,2,3-tribromopropyl group, an iodomethylgroup, a 1-iodoethyl group, a 2-iodoethyl group, a 2-iodoisobutyl group,a 1,2-diiodoethyl group, a 1,3-diiodoisopropyl group, a2,3-diiodo-t-butyl group, a 1,2,3-triiodopropyl group, an aminomethylgroup, a 1-aminoethyl group, a 2-aminoethyl group, a 2-aminoisobutylgroup, a 1,2-diaminoethyl group, a 1,3-diaminoisopropyl group, a2,3-diamino-t-butyl group, a 1,2,3-triaminopropyl group, a cyanomethylgroup, a 1-cyanoethyl group, a 2-cyanoethyl group, a 2-cyanoisobutylgroup, a 1,2-dicyanoethyl group, a 1,3-dicyanoisopropyl group, a2,3-dicyano-t-butyl group, a 1,2,3-tricyanopropyl group, a nitromethylgroup, a 1-nitroethyl group, a 2-nitroethyl group, a 2-nitroisobutylgroup, a 1,2-dinitroethyl group, a 1,3-diiodoisopropyl group, a2,3-dinitro-t-butyl group, and a 1,2,3-trinitropropyl group. Amongthese, a methyl group, an ethyl group, a propyl group, an isopropylgroup, a n-butyl group, a s-butyl group, an isobutyl group, and at-butyl group are preferable.

Examples of the substituted or unsubstituted cycloalkyl groups havingfrom 3 to 50 ring-forming carbon atoms of R¹ to R⁸, R¹¹, and Ar⁵ to Ar⁷include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, a 4-methylcyclohexyl group, a 1-adamantyl group, a2-adamantyl group, a 1-norbornyl group, and a 2-norbornyl group. Amongthese, a cyclopentyl group and a cyclohexyl group are preferable.

Examples of the substituted or unsubstituted alkoxy groups having from 1to 50 carbon atoms of R¹ to R⁸ and R¹¹ include a group represented by—OZ. Z is selected from the substituted or unsubstituted alkyl grouphaving from 1 to 50 carbon atoms of R¹ to R⁸ described above.

Examples of the substituted or unsubstituted aralkyl groups having from7 to 50 carbon atoms (wherein an aryl moiety has from 6 to 49 carbonatoms and an alkyl moiety has from 1 to 44 carbon atoms) of R¹ to R⁸,R¹¹, and Ar⁵ to Ar⁷ include a benzyl group, a 1-phenylethyl group, a2-phenylethyl group, a 1-phenylisopropyl group, a 2-phenylisopropylgroup, a phenyl-t-butyl group, an α-naphthylmethyl group, a1-α-naphthylethyl group, a 2-α-naphthylethyl group, a1-α-naphthylisopropyl group, a 2-α-naphthylisopropyl group, aβ-naphthylmethyl group, a 1-β-naphthylethyl group, a 2-β-naphthylethylgroup, a 1-β-naphthylisopropyl group, a 2-β-naphthylisopropyl group, a1-pyrrolylmethyl group, a 2-(1-pyrrolyl)ethyl group, a p-methylbenzylgroup, a m-methylbenzyl group, an o-methylbenzyl group, a p-chlorobenzylgroup, a m-chlorobenzyl group, an o-chlorobenzyl group, a p-boromobenzylgroup, a m-boromobenzyl group, an o-boromobenzyl group, a p-iodobenzylgroup, a m-iodobenzyl group, an o-iodobenzyl group, a p-hydroxybenzylgroup, a m-hydroxybenzyl group, an o-hydroxybenzyl group, ap-aminobenzyl group, a m-aminobenzyl group, an o-aminobenzyl group, ap-nitrobenzyl group, a m-nitrobenzyl group, an o-nitrobenzyl group, ap-cyanobenzyl group, a m-cyanobenzyl group, an o-cyanobenzyl group, a1-hydroxy-2-phenylisopropyl group, and a 1-chloro-2-phenylisopropylgroup.

The substituted or unsubstituted aryloxy groups having from 6 to 50ring-forming carbon atoms of R¹ to R⁸, and R¹¹ are a group representedby —OY, and the arylthio group is a group represented by —SY. Y isselected from the substituted or unsubstituted aryl groups having from 6to 50 ring-forming carbon atoms of R¹ to R⁸.

The substituted or unsubstituted alkoxycarbonyl groups having from 2 to50 carbon atoms (the alkyl moiety has 1 to 49 carbon atoms) of R¹ to R⁸and R¹¹ are a group represented by —COOZ. Z is selected from thesubstituted or unsubstituted alkyl groups having from 1 to 50 carbonatoms of R¹ to R⁸.

Examples of the substituted silyl groups of R¹ to R⁸ and R¹¹ include atrimethylsilyl group, a triethylsilyl group, a t-butyldimethylsilylgroup, a vinyldimethylsilyl group, a propyldimethylsilyl group, and atriphenylsilyl group.

Examples of the halogen atoms of R¹ to R⁸ and R¹¹ include fluorine,chlorine, bromine, and iodine.

Examples of the aminoanthracene derivative used as a dopant include acompound represented by Formula (11) below.

Such a compound is used as a dopant.

In Formula (11), A₁ and A₂ are each independently a substituted orunsubstituted aliphatic hydrocarbon group having from 1 to 6ring-forming carbon atoms, a substituted or unsubstituted aromatichydrocarbon group having from 6 to 20 carbon atoms, or a substituted orunsubstituted heteroaromatic hydrocarbon group having from 5 to 19carbon atoms, containing any of a nitrogen atom, a sulfur atom, and anoxygen atom. Each A₃ is independently a substituted or unsubstitutedaliphatic hydrocarbon group having from 1 to 6 carbon atoms, asubstituted or unsubstituted aromatic hydrocarbon group having from 6 to20 carbon atoms, a substituted or unsubstituted heteroaromatichydrocarbon group having from 5 to 19 carbon atoms, containing any of anitrogen atom, a sulfur atom, and an oxygen atom, or a hydrogen atom.

Examples of the aminochrysene derivative include a compound representedby Formula (12) below.

Such a compound is preferable.

In Formula (12), X₁ to X₁₀ are each independently a hydrogen atom or asubstituent, and Y₁ and Y₂ are each independently a substituent. X₁ toX₁₀ are preferably a hydrogen atom. Y₁ and Y₂ are each independentlypreferably a substituted (preferably substituted with an alkyl grouphaving from 1 to 6 carbon atoms) or unsubstituted aromatic ring(preferably an aromatic ring having from 6 to 10 carbon atoms, or aphenyl group) having from 6 to 30 carbon atoms.

Examples of the aminopyrene derivative include a compound represented byFormula (13) below.

Such a compound is preferable.

In Formula (13), X₁ to X₁₀ are each independently a hydrogen atom or asubstituent. Provided that X₃ and X₈ or X₂ and X₇ are each a —NY₁Y₂group, wherein Y₁ and Y₂ are a substituent. When X₃ and X₈ are each—NY₁Y₂ group, it is preferable that X₂, X₄, X₅, X₇, X₉, and X₁₀ be ahydrogen atom and X₁ and X₆ be a hydrogen atom, an alkyl group, or acycloalkyl group. When X₂ and X₇ are each —NY₁Y₂ group, it is preferablethat X₁, X₃ to X₆, and X₈ to X₁₀ be a hydrogen atom. Y₁ and Y₂ arepreferably a substituted aromatic ring (e.g., substituted with an alkylgroup having from 1 to 6 carbon atoms) or unsubstituted aromatic ring(e.g., a phenyl group and a naphthyl group).

The content (doping amount) of the TTF material in the firstlight-emitting layer 33 a is preferably not less than 10 wt %, morepreferably not less than 30 wt %, and most preferably not less than 50wt %.

When the host material is used, it is desirable that the T₁ level of thehost material be higher than the T₁ level of the TTF material. In thiscase, excitation energy is easily transferred from the host material tothe TTF material.

In the present embodiment, it is preferable that the firstlight-emitting layer 33 a have high hole transport property and thesecond light-emitting layer 33 b and the third light-emitting layer 33 chave high electron transport property. Therefore, it is desirable thatthe first light-emitting layer 33 a contain a material having a holemobility higher than the electron mobility and the second light-emittinglayer 33 b and the third light-emitting layer 33 c contain a materialhaving an electron mobility higher than the hole mobility.

When such materials are used, holes are efficiently injected ortransported from the positive electrode 2 to the first light-emittinglayer 33 a, and electrons are efficiently injected or transported fromthe negative electrode 4 to the second light-emitting layer 33 b throughthe third light-emitting layer 33 c. As a result, recombination ofcarriers (electrons and holes) is likely to occur in the vicinity ofinterface between the first light-emitting layer 33 a and the secondlight-emitting layer 33 b. Therefore, a Dexter transition from thesecond light-emitting layer 33 b to the first light-emitting layer 33 ais likely to occur.

Accordingly, when the first light-emitting layer 33 a contains the hostmaterial, it is desirable that as the host material, the material havinga hole mobility higher than the electron mobility be used.

The thickness of the first light-emitting layer 33 a is preferablygreater than 5 nm, and more preferably less than 5 nm.

In the present embodiment, light emission of the first light-emittinglayer 33 a is performed by using Dexter energy transfer of excitonsgenerated in the second light-emitting layer 33 b. A Dexter transitionoccurs only between adjacent molecules. Therefore, when the thickness ofthe first light-emitting layer 33 a is large, the light-emittingefficiency may be decreased. Accordingly, it is desirable that thethickness of the first light-emitting layer 33 a be not greater than 5nm.

Second Light-Emitting Layer 33 b

For the second light-emitting layer 33 b, a TADF material is used as thedelayed fluorescent material. The second light-emitting layer 33 bcontains a host material (second host material) and the TADF material,or at least the TADF material.

The TADF material is a material in which a singlet excited state (S₁)can be generated by reverse intersystem crossing from a triplet excitedstate (T₁) by thermal activation. The TADF material is a delayedfluorescent material in which the energy difference Δ_(EST) between theS₁ level and the T₁ level is extremely small. When the delayedfluorescent material in which the energy difference Δ_(EST) between theS₁ level and the T₁ level is extremely small is used for the dopantmaterial, reverse intersystem crossing from the T₁ level to the S₁ leveldue to thermal energy occurs. It is considered that use of delayedfluorescence due to the TADF material can theoretically enhance theinternal quantum efficiency to 100% in fluorescent emission.

As the TADF material, a known material can be used, and is notparticularly limited. Examples of green light-emitting TADF materialused in the present embodiment include phenoxazine-triphenyltriazine(PXZ-TRZ). The TADF material may be used alone, or two or more kindsthereof may be appropriately used in combination.

For the TADF material, a material having a higher T₁ level than the T₁level of at least one of the host material and the TTF materialcontained in the first light-emitting layer 33 a is selected. In thiscase, energy is easily transferred from the TADF material to the atleast one of the host material and the TTF material contained in thefirst light-emitting layer 33 a.

As the host material, a publicly known host material that has beentypically used for an EL layer (in the present embodiment, the organicEL layer) can be used. Examples of the host material include acene-basedmaterials such as a carbazole derivative, anthracene, and tetracene, andderivatives thereof. One kind of the host material may be used alone, ortwo or more kinds of thereof may be appropriately used in combination.

When the host material is used, it is desirable that the T₁ level of thehost material be higher than the S₁ level and the T₁ level of the TADFmaterial. In this case, excitation energy is easily transferred from thehost material to the TADF material.

In the present embodiment, it is preferable that the firstlight-emitting layer 33 a have high hole-transporting property and thesecond light-emitting layer 33 b and the third light-emitting layer 33 chave high electron-transporting property, as described above.

Therefore, as the host material, an electron-transporting host materialhaving an electron mobility higher than the hole mobility is preferablyused. Accordingly, as the host material, for example, anelectron-transporting material such as2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, trivial name:bathocuproine) is preferably used.

The content (doping amount) of the TADF material in the secondlight-emitting layer 33 b is preferably not less than 10 wt %, morepreferably not less than 30 wt %, and most preferably not less than 50wt %. Therefore, when the second light-emitting layer 33 b contains thehost material, the host material is doped with the TADF material suchthat the second light-emitting layer 33 b contains the TADF material ina content of at least 10%.

It is desirable that the HOMO level of a material contained at least atthe highest mixing ratio in the second light-emitting layer 33 b behigher than the HOMO level of a material contained at least at thehighest mixing ratio in the first light-emitting layer 33 a. In thiscase, holes are more easily injected into the second light-emittinglayer 33 b than the first light-emitting layer 33 a.

It is desirable that the HOMO level of the material contained at leastat the highest mixing ratio in the second light-emitting layer 33 b behigher than the HOMO level of a material contained at least at thehighest mixing ratio in the third light-emitting layer 33 c. In thiscase, holes are hardly leaked from the second light-emitting layer 33 bto the third light-emitting layer 33 c.

It is desirable that the LUMO level of the material contained at leastat the highest mixing ratio in the second light-emitting layer 33 b belower than the LUMO level of the material contained at least at thehighest mixing ratio in the first light-emitting layer 33 a. In thiscase, electrons are hardly leaked from the second light-emitting layer33 b to the first light-emitting layer 33 a.

It is desirable that the LUMO level of the material contained at leastat the highest mixing ratio in the second light-emitting layer 33 b belower than the LUMO level of the material contained at least at thehighest mixing ratio in the third light-emitting layer 33 c. In thiscase, electrons are more easily injected into the second light-emittinglayer 33 b than the third light-emitting layer 33 c.

Therefore, when materials satisfying the relationships described aboveare used for the first light-emitting layer 33 a, the secondlight-emitting layer 33 b, and the third light-emitting layer 33 c, theexcitation generation probability in the second light-emitting layer 33b can be enhanced. As a result, the light-emitting efficiency of thesecond light-emitting layer 33 b can be improved. In addition, due totransfer of energy of excitations generated in the second light-emittinglayer 33 b, light can be efficiently emitted also by the firstlight-emitting layer 33 a and the third light-emitting layer 33 c.

The threshold value of photoelectrons that are released by irradiationwith ultraviolet light by using an atmospheric pressure photoemissionspectrometer “AC-3” available from Riken Keiki Co., Ltd., or the like,is measured. Thus, the HOMO level can be determined. On the other hand,a band gap is determined from the energy of an absorption edge of anabsorption spectrum under irradiation with ultraviolet light by using anultraviolet-visible spectroscopy “UV-2450” available from ShimadzuCorporation. The LUMO level can be determined by calculation from theband gap and the HOMO level determined by the procedure described above.

For comparison of the HOMO level and the LUMO level, an energydifference is important, but not values themselves. Both the HOMO leveland the LUMO level can be determined by an ordinary procedure.Therefore, comparison of the HOMO levels and comparison of the LUMOlevels that are determined by the same procedure is desirable althoughdetails of a measurement method are omitted.

The thickness of the second light-emitting layer 33 b is preferably notgreater than 20 nm, more preferably from 2 to 20 nm, and even morepreferably from 5 to 10 nm.

In the present embodiment, the third light-emitting layer 33 c is madeto emit light by using a Förster transition (Förster energy transfer)from the TADF material in the second light-emitting layer 33 b to afluorescent material in the third light-emitting layer 33 c.

The Förster transition occurs when the materials are within a constantdistance without direct contact. The second light-emitting layer 33 b isa green light-emitting layer (green-fluorescence emitting layer), andthe third light-emitting layer 33 c is a red light-emitting layer(red-fluorescence emitting layer). Therefore, the energy levels of thesecond light-emitting layer 33 b and the third light-emitting layer 33 chave a relationship of S_(1G)>S_(1R) and T_(1G)>T_(1R). Accordingly,energy is easily transferred from the second light-emitting layer 33 bto the third light-emitting layer 33 c. When a distance between amolecule of the TADF material and a molecule of the fluorescent materialin the third light-emitting layer 33 c is not greater than 20 nm, theFörster transition easily and certainly occurs, and the energy transferefficiency is not deteriorated.

When the thickness of the second light-emitting layer 33 b is notgreater than 20 nm, the distance between the third light-emitting layer33 c and the molecule of the TADF material that is positioned thefarthest from the fluorescent material in the third light-emitting layer33 c (i.e., a surface of the second light-emitting layer 33 b on a sideopposite to the third light-emitting layer 33 c, that is, a surface onthe side of the first light-emitting layer 33 a) in the secondlight-emitting layer 33 b is not greater than 20 nm. Therefore, theshortest distances between an optional position of the secondlight-emitting layer 33 b and the third light-emitting layer 33 c areall not greater than 20 nm. Accordingly, even in the molecule of theTADF material positioned on the surface of the second light-emittinglayer 33 b on the side opposite to the third light-emitting layer 33 c,the Förster transition may occur. The Förster transition from themolecule of the TADF material at the optional position to thefluorescent material in the third light-emitting layer 33 c may occur.

To make the thicknesses of the layers uniform and suppress a decrease inlight-emitting efficiency, the thickness of the second light-emittinglayer 33 b is preferably not less than 2 nm, and more preferably notless than 5 nm.

Third Light-Emitting Layer 33 c

As described above, the fluorescent material having the longest emissionpeak wavelength of the light-emitting materials used in thelight-emitting layer 33 is used for the third light-emitting layer 33 c.The third light-emitting layer 33 c contains the host material (thirdhost material) and the fluorescent material, or at least the fluorescentmaterial.

As the fluorescent material, a known fluorescent material can be used,and is not particularly limited as long as it is a fluorescent materialhaving a S₁ level lower than the S₁ level and the T₁ level of the TADFmaterial in the second light-emitting layer 33 b. Examples of a redlight-emitting fluorescent material used in the present embodimentinclude a 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran(DCM), a 4-dicyanomethylene-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran(DCM2), a perylene derivative such as a diindenoperylene derivative, aneuropium complex, a benzopyran derivative, a rhodamine derivative, abenzothioxanthene derivative, a porphyrin derivative, and nile red.

As the host material, a publicly known host material that has beentypically used for an EL layer (in the present embodiment, the organicEL layer) can be used. As the host material, for example, the same hostmaterial as the host material used in the second light-emitting layer 33b can be used. Specific examples of the host material includeacene-based materials such as a carbazole derivative, anthracene, andtetracene, and derivatives thereof. One kind of the host material may beused alone, or two or more kinds of thereof may be appropriately used incombination.

In the present embodiment, it is preferable that the secondlight-emitting layer 33 b and the third light-emitting layer 33 c havehigh electron-transporting property, as described above. Therefore, itis preferable that an electron-transporting host material having anelectron mobility higher than the hole mobility be used for the thirdlight-emitting layer 33 c. Accordingly, as the host material, forexample, an electron-transporting material such as bathocuproine (BCP)is preferably used.

The content (doping amount) of the fluorescent material in the thirdlight-emitting layer 33 c is preferably not less than 10 wt %, morepreferably not less than 30 wt %, and most preferably not less than 50wt %.

The thickness of the third light-emitting layer 33 c is preferably notgreater than 20 nm, and more preferably from 10 to 20 nm.

The Förster radius is approximately 15 nm. Therefore, for all moleculesof the third light-emitting layer 33 c having a thickness of not greaterthan 20 nm, a Förster transition occurs. In the third light-emittinglayer 33 c having a thickness of not less than 10 nm, energy transferfrom the third light-emitting layer 33 c to a layer on a side oppositeto the second light-emitting layer 33 b hardly occurs. Therefore, whenthe thickness of the third light-emitting layer 33 c is from 10 to 20nm, the light-emitting efficiency is the highest.

Hole Injecting Layer 31 and Hole Transport Layer 32

The hole injecting layer 31 includes a hole-injecting material and has afunction to increase the hole injection efficiency to the light-emittinglayer 33.

The hole transport layer 32 includes a hole-transporting material andhas a function to increase the hole transport efficiency to thelight-emitting layer 33.

The hole injecting layer 31 and the hole transport layer 32 may beformed as mutually independent layers, or may be integrated together asa hole injection-cum-transport layer. It is not necessary that both thehole injecting layer 31 and the hole transport layer 32 be provided.Only one of the hole injecting layer 31 and the hole transport layer 32,for example, only the hole transport layer 32 may be provided. Both thehole injecting layer 31 and the hole transport layer 32 may not beprovided.

As a material for the hole injecting layer 31, the hole transport layer32, or the hole injection-cum-transport layer, that is, a material usedas the hole-injecting material or the hole-transporting material, aknown material can be used.

Examples of the material include linear or heterocyclic conjugatedmonomers, oligomers, or polymers such as naphthalene, anthracene,azatriphenylene, fluorenone, hydrazone, stilbene, triphenylene, benzine,styrylamine, triphenylamine, porphyrins, triazole, imidazole,oxadiazole, oxazole, polyarylalkane, phenylenediamine, arylamine, andderivative thereof, a thiophene-based compound, a polysilane-basedcompound, a vinyl carbazole-based compound, and an aniline-basedcompound.

Specifically, N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (α-NPD),2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN),1,3-bis(carbazol-9-yl)benzene (mCP),di[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC),9,10-diphenylanthracene-2-sulfonate (DPAS),N,N′-diphenyl-N,N′-(4-(di(3-tolyl)amino)phenyl)-1,1′-biphenyl-4,4′-diamine(DNTPD), iridium (III) tris[N,N′-diphenylbenzimidazol-2-ylidene-C2,C2′](Ir(dpbic)₃), 4,4′,4″-tris-(N-carbazolyl)-triphenylamine (TCTA),2,2-bis(p-trimellitic oxyphenyl)propanoic anhydride (BTPD),bis[4-(p,p-ditolylamino)phenyl]diphenylsilane (DTASi), or the like isused.

For the hole injecting layer 31, the hole transport layer 32, or thehole injection-cum-transport layer, an intrinsic hole-injecting materialor an intrinsic hole-transporting material that is not doped with animpurity may be used. Alternatively, the material may be doped with animpurity to enhance the electrical conductivity.

To obtain highly-efficient light emission, it is desirable that theexcitation energy be trapped within the light-emitting layer 33.Therefore, it is desirable that as the hole-injecting material and thehole-transporting material, a material having an S₁ level and a T₁ levelthat are excitation levels higher than the S₁ level and the T₁ level ofthe fluorescent material (dopant material) in the light-emitting layer33 be used. Therefore, it is more preferable that as the hole-injectingmaterial and the hole-transporting material, a material having highexcitation level and high hole mobility be selected.

Electron Transport Layer 34 and Electron Injecting Layer 35

The electron injecting layer 35 includes an electron-injecting materialand has a function to increase the electrode injection efficiency to thelight-emitting layer 33.

The electron transport layer 34 includes an electron-transportingmaterial and has a function to increase the electron transportefficiency to the light-emitting layer 33.

The electron injecting layer 35 and the electron transport layer 34 maybe formed as mutually independent layers, or may be integrated togetheras an electron injection-cum-transport layer. It is not necessary thatboth the electron injecting layer 35 and the electron transport layer 34be provided. Only one of the electron injecting layer 35 and theelectron transport layer 34, for example, only the electron transportlayer 34 may be provided. Both the electron injecting layer 35 and theelectron transport layer 34 may not be provided.

As a material for the electron injecting layer 35, the electrontransport layer 34, or the electron injection-cum-transport layer, thatis, a material used as the electron-injecting material or theelectron-transporting material, a known material can be used.

Examples of the material include quinoline, perylene, phenanthroline,bistyryl, pyrazine, triazole, oxazole, oxadiazole, fluorenone, andderivatives and metal complexes thereof, and lithium fluoride (LiF).

Specific examples thereof include 4,7-diphenyl-1,10-phenanthroline(Bphen), 3,3′-bis(9H-carbazol-9-yl)biphenyl (mCBP),2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP),1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI),3-phenyl-4(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ),1,10-phenanthroline, Alq(tris(8-hydroxyquinoline) aluminum), and LiF.

For the electron injecting layer 35, the electron transport layer 34, orthe electron injection-cum-transport layer, an intrinsicelectron-injecting material or an intrinsic electron-transportingmaterial that is not doped with an impurity may be used. Alternatively,the material may be doped with an impurity to enhance the electricalconductivity.

To obtain highly-efficient light emission, it is desirable that theexcitation energy be trapped within the light-emitting layer 33.Therefore, it is desirable that as the electron-injecting material andthe electron-transporting material, a material having an S₁ level and aT₁ level that are excitation levels higher than the S₁ level and the T₁level of the dopant material in the adjacent light-emitting layer 33 beused. Therefore, it is more preferable that as the electron-injectingmaterial and the electron-transporting material, a material having highexcitation level and high electron mobility be selected.

The thicknesses of the layers formed as necessary, other than thelight-emitting layer 33 is appropriately set such that excitons aregenerated in the second light-emitting layer 33 b, but is notparticularly limited. The thicknesses may be set according to themobility of carriers (holes and electrons) in each layer, the balancethereof, or the kind of material constituting each layer. Thethicknesses of the layers can be set similarly to those in the knownorganic EL element.

Principle of Light Emission

Next, the principle of light emission of the organic EL element 10according to the present embodiment will be described hereinafter withreference to FIGS. 1A and 1B.

In the organic EL element 10 according to the present embodiment, lightcan be emitted by using the following features. (1) When light-emittinglayers having different emission peak wavelengths are layered, theexcitation energy is easily transferred from the light-emitting layer ofwhich the emission peak wavelength is on the shorter wavelength side tothe light-emitting layer of which the emission peak wavelength is on thelonger wavelength side. (2) Since in the TADF material, excitons can befreely transferred between the S₁ level and the T₁ level, excitons aregenerated uniformly in the S₁ level and the T₁ level of the TADFmaterial. (3) The TTF material can be made to emit light byre-excitation from the T₁ level to the S₁ level.

The organic EL element 10 has a configuration in which the firstlight-emitting layer 33 a (blue light-emitting layer) having theshortest emission peak wavelength of the light-emitting layer 33, thesecond light-emitting layer 33 b (green light-emitting layer), and thethird light-emitting layer 33 c (red light-emitting layer) having thelongest emission peak wavelength of the light-emitting layer 33 arelayered in this order between a pair of electrodes (the positiveelectrode 2 and the negative electrode 4). The first light-emittinglayer 33 a contains the host material and the TTF material or at leastthe TTF material. The second light-emitting layer 33 b contains the hostmaterial and the TADF material or at least the TADF material. The thirdlight-emitting layer 33 c contains the host material and the fluorescentmaterial or at least the fluorescent material. The T₁ level of at leastone of the host material and the TTF material contained in the firstlight-emitting layer 33 a is lower than the T₁ level of the TADFmaterial contained in the second light-emitting layer 33 b.

Even when excitons are generated in a general fluorescent material, asdescribed above, 25% of singlet excitons can be only used in lightemission.

However, in the TADF material, an energy difference Δ_(EST) between theS₁ level and the T₁ level is extremely small, as illustrated in FIG. 1A.The TADF material is a delayed fluorescent material in which a singletexcited state (S₁) can be generated by reverse intersystem crossing froma triplet excited state (T₁) by thermal activation. Therefore, light canbe emitted by returning triplet excitons into the singlet excitons. Itis considered that use of delayed fluorescence of the TADF material cantheoretically enhance the internal quantum efficiency to 100% influorescent emission, as described above.

As described in (2), in the TADF material, excitons can be freelytransferred between the S₁ level and the T₁ level. Therefore, excitonsare generated uniformly in the S₁ level and the T₁ level of thethermally activated delayed fluorescent material.

As the emission peak wavelength is shorter, the energy level of thelight-emitting material is higher. The second light-emitting layer 33 bis a green light-emitting layer, and the third light-emitting layer 33 cis a red light-emitting layer. Therefore, the energy levels of thesecond light-emitting layer 33 b and the third light-emitting layer 33 chave a relationship of S_(1G)>S_(1R) and T_(1G)>T_(1R), as illustratedin FIG. 1A. Accordingly, energy is easily transferred from the secondlight-emitting layer 33 b that is the green light-emitting layer to thethird light-emitting layer 33 c that is the red light-emitting layer, asdescribed in (1).

According to the present embodiment, Förster energy transfer (Förstertransition) from the TADF material in the second light-emitting layer 33b to the fluorescent material in the third light-emitting layer 33 chaving a S₁ level lower than the TADF material occurs, as illustrated inFIGS. 1A and 1B. According to the present embodiment, light emission canbe efficiently achieved not only in the second light-emitting layer 33 bbut also in the third light-emitting layer 33 c.

Further, in the organic EL element 10, the T₁ level of at least one ofthe host material and the TTF material contained in the firstlight-emitting layer 33 a (e.g., the T₁ level of the TTF material, asillustrated in FIG. 1A) is lower than the T₁ level of the TADF materialcontained in the second light-emitting layer 33 b. Therefore, excitationenergy is easily transferred from the TADF material in the secondlight-emitting layer 33 b to the host material or the TTF materialcontained in the first light-emitting layer 33 a.

At that time, Dexter energy transfer from the TADF material in thesecond light-emitting layer 33 b to the host material or the TTFmaterial in the first light-emitting layer 33 a occurs, as illustratedin FIGS. 1A and 1B. This is because the second light-emitting layer 33 bis layered on the first light-emitting layer 33 a (that is, the secondlight-emitting layer 33 b is in contact with the first light-emittinglayer 33 a). Subsequently, due to the TTF phenomenon, re-excitation fromthe T₁ level of the host material or the T₁ level of the TTF material tothe S₁ level of the TTF material occurs in the first light-emittinglayer 33 a, as illustrated in (3). According to the present embodiment,light emission can be efficiently achieved also in the firstlight-emitting layer 33 a.

Therefore, according to the present embodiment, use of a fluorescentmaterial containing at least two kinds of delayed fluorescent materialsfor the light-emitting layer 33, as described above, can achieveemission of lights of three colors with efficiency.

As described above, in the organic EL element 10, the fluorescentmaterial containing the delayed fluorescent material is used to emitlight, and a phosphorescent material is not used. Therefore, a reductionin cost can be attained. Accordingly, according to the presentembodiment, a highly-efficient white light-emitting device usingthree-color light emission at low cost can be provided.

When Dexter energy transfer from the T₁ level of the TADF material inthe second light-emitting layer 33 b directly to the T₁ level of the TTFmaterial in the first light-emitting layer 33 a occurs, light emissioncan be more efficiently archived in the first light-emitting layer 33 a.When Förster energy transfer from the T₁ level of the TADF material inthe second light-emitting layer 33 b directly to the T₁ level of thefluorescent material in the third light-emitting layer 33 c occurs,light emission can be more efficiently archived in the thirdlight-emitting layer 33 c.

When the electrode on the side of the first light-emitting layer 33 a isthe positive electrode 2 and the electrode on the side of the thirdlight-emitting layer 33 c is the negative electrode 4, like the presentembodiment, holes are efficiently injected or transported from thepositive electrode 2 to the first light-emitting layer 33 a, andelectrons are efficiently injected or transported from the negativeelectrode 4 to the second light-emitting layer 33 b through the thirdlight-emitting layer 33 c. This is because the first light-emittinglayer 33 a has high hole-transporting property and the secondlight-emitting layer 33 b and the third light-emitting layer 33 c havehigh electron-transporting property. Therefore, recombination ofcarriers is likely to occur in the vicinity of the interface between thefirst light-emitting layer 33 a and the second light-emitting layer 33b, as illustrated in FIG. 1B. Accordingly, the exciton generationprobability in the second light-emitting layer 33 b is increased, and aDexter transition from the second light-emitting layer 33 b to the firstlight-emitting layer 33 a is likely to occur. As described above, theDexter transition occurs due to direct contact of materials (molecules).However, a Förster transition from the S₁ level of the TADF material tothe S₁ level of the fluorescent material occurs when the materials arewithin a constant distance without direct contact. Therefore, thisconfiguration makes it possible to emit lights of three colors withefficiency.

Method for Manufacturing Organic EL Element 10

A method for manufacturing the organic EL element 10 includes the stepsof forming a first electrode that is one of the pair of electrodes (thepositive electrode 2 and the negative electrode 4), forming the organicEL layer 3 (functional layer forming step), and forming a secondelectrode that is the other of the pair of electrodes.

In a process of manufacturing the organic EL element 10, the organic ELlayer forming step is performed between the step of forming the firstelectrode and the step of forming the second electrode. Thus, theorganic EL layer 3 is formed between the first electrode and the secondelectrode. Therefore, the step of forming the organic EL layer 3 betweenthe first electrode and the second electrode includes at least steps offorming the first light-emitting layer 33 a (first light-emitting layerforming step), forming the second light-emitting layer 33 b (secondlight-emitting layer forming step), and forming the third light-emittinglayer 33 c (third light-emitting layer forming step).

The first light-emitting layer forming step and the secondlight-emitting layer forming step are continuously performed such thatthe second light-emitting layer 33 b is layered between the firstlight-emitting layer 33 a and the third light-emitting layer 33 c andthe first light-emitting layer 33 a and the second light-emitting layer33 b are adjacent to each other.

In the present embodiment, the first electrode is the positive electrode2, the organic EL layer forming step includes forming the hole injectinglayer 31 on the positive electrode 2, forming the hole transport layer32 on the hole injecting layer 31, forming the first light-emittinglayer 33 a on the hole injecting layer 31, forming the secondlight-emitting layer 33 b on the first light-emitting layer 33 a,forming the third light-emitting layer 33 c on the second light-emittinglayer 33 b, forming the electron transport layer 34 on the thirdlight-emitting layer 33 c, and forming the electron injecting layer 35on the electron transport layer 34, and the negative electrode 4 isformed as the second electrode on the electron injecting layer 35.

For example, an electrically conductive film (electrode film) is formedon the substrate 1, a photoresist is applied to the electricallyconductive film, patterning is performed by photolithography, theelectrically conductive film is etched, and the photoresist is peeled.Thus, the first electrode can be formed.

In layering the electrically conductive film, a sputtering method, a CVDmethod, a plasma CVD method, a printing method, or the like can beemployed.

In the present embodiment, the non-translucent electrode 21 and thelight-transmitting electrode 22 are formed as the positive electrode 2,as illustrated in FIG. 2.

The functional layers constituting the organic EL layer 3, such as thehole injecting layer 31, the hole transport layer 32, the firstlight-emitting layer 33 a, the second light-emitting layer 33 b, thethird light-emitting layer 33 c, the electron transport layer 34, andthe electron injecting layer 35, can be formed by various types ofprocedures.

For example, the layers can be formed by a publicly known wet process bya coating method such as a spin coating method, a dipping method, adoctor blade method, a discharge coating method, and a spray coatingmethod, or a printing method such as an inkjet method, a letterpressprinting method, an intaglio printing method, a screen printing method,and a microgravure-coating method. In the process, a coating liquid forformation of the organic EL layer in which the materials for the layersare dissolved and dispersed in a solvent is used.

The layers can be also formed by a publicly known dry process by aresistive heating vapor deposition method, an electron beam (EB) vapordeposition method, a molecular beam epitaxy (MBE) method, a sputteringmethod, an organic vapor phase deposition (OVPD) method, or the like,using the materials. The host material can be doped with the dopantmaterial by co-evaporation deposition of the dopant material and thehost material.

Alternatively, the layers can be formed by a laser transfer method usingthe materials.

When the organic EL layer 3 is formed by a wet process, the coatingliquid for formation of the organic EL layer may contain an additive foradjusting the physical properties of the coating liquid, such as aleveling agent and a viscosity adjusting agent.

In formation of the second electrode, a sputtering method, a vacuumvapor deposition method, a CVD method, a plasma CVD method, a printingmethod, or the like can be employed. Patterning may be performed byphotolithography.

Hereinafter, the present embodiment will be described in detail withreference to Examples. In the following Examples, a part of componentswill be described by using specific dimensions and materials asexamples. However, the present embodiment is not limited to the specificdimensions and materials. That is, the present embodiment is not limitedto the following Examples.

Example 1

In Example 1, the non-translucent electrode 21, the light-transmittingelectrode 22, the hole injecting layer 31, the hole transport layer 32,the first light-emitting layer 33 a, the second light-emitting layer 33b, the third light-emitting layer 33 c, the electron transport layer 34,the electron injecting layer 35, and the negative electrode 4 werelayered in this order on the substrate 1, as illustrated in FIG. 2.

As the substrate 1, a glass substrate was used. Materials for the layerslayered on the substrate 1 and thicknesses thereof are as follows.

Non-translucent electrode 21 (positive electrode 2, reflectiveelectrode): Ag (100 nm)

Light-transmitting 22 (positive electrode 2): ITO (65 nm)

Hole injecting layer 31: HAT-CN (10 nm)

Hole transport layer 32: α-NPD (20 nm)

First light-emitting layer 33 a (blue light-emitting layer,light-emitting layer 33): BH1 (host material, 90%)/BD1 (TTF material,10%) (5 nm)

Second light-emitting layer 33 b (green light-emitting layer,light-emitting layer 33): BCP (host material, 50%)/PXZ-DPS (TADFmaterial, 50%) (10 nm)

Third light-emitting layer 33 c (red light-emitting layer,light-emitting layer 33): BCP (host material, 90%)/DCM (fluorescentmaterial, 10%) (20 nm)

Electron transport layer 34: Bphen (30 nm)

Electron injecting layer 35: LiF (1 nm)

Negative electrode 4 (translucent electrode): Ag—Mg alloy (Ag/Mg mixingratio=0.9/0.1) (20 nm)

For the first light-emitting layer 33 a, as a dopant material (TTFmaterial) and a host material that cause a TTF phenomenon, a combinationof delayed fluorescent materials that cause TTF from the T₁ level of BH1that is the host material to the S₁ level of BD1 that is the dopantmaterial (TTF material) was used. Specifically, of the combination ofthe dopant material and the host material that cause a TTF phenomenon, ahost material that satisfies the conditions described above forcarrier-transporting properties (hole-transporting property andelectron-transporting property) and energy levels (HOMO level, LUMOlevel, T₁ level, and S₁ level) was selected, and a dopant materialsuitable for the host material was selected.

The T₁ level of BH1 is lower than the T₁ level of PXZ-DPS that is thedopant material (TADF material) of the second light-emitting layer 33 b.The S₁ level of BD1 is higher than the T₁ level of BH1.

In Example 1, since the T₁ level of BH1 is lower than the T₁ level ofthe TADF material, Dexter energy transfer from the TADF material to atleast one of BH1 and BD1 occurs, and due to TTF, upconversion from theT₁ level of the BH1 or the T₁ level of BD1 to the S₁ level of BD1occurs. In the present embodiment, BD1 having a S₁ level higher than theS₁ level of the dopant material (TADF material) of the secondlight-emitting layer 33 b is made to emit light by using delayedfluorescence due to the TTF phenomenon, as described above.

At that time, the T₁ level of BD1 is higher than the T₁ level of BH1.Therefore, triplet excitons of BH1 are not transferred to BD1, tripletexcitons generated in BD1 are transferred as energy to BH1, and due tothe TTF phenomenon, the triplet excitons collide with each other on BH1,to generate singlet excitons. The singlet excitons generated due to theTTF phenomenon are transferred as energy from BH1 to BD1 that emitslight, and contribute to fluorescence emission of BD1.

As described above, in the organic EL element 10 according to thisExample, the first light-emitting layer 33 a is a blue light-emittinglayer, the second light-emitting layer 33 b is a green light-emittinglayer, and the third light-emitting layer 33 c is a red light-emittinglayer. Further, the first light-emitting layer 33 a contains a delayedfluorescent material having the shortest wavelength of the fluorescentmaterials used for the light-emitting layer 33, and the thirdlight-emitting layer 33 c contains a fluorescent material a fluorescentmaterial having the longest wavelength of the fluorescent materials usedfor the light-emitting layer 33. Moreover, the second light-emittinglayer 33 b contains the TADF material, the proportion of the TADFmaterial to be mixed in the second light-emitting layer 33 b is high,and the thickness of the second light-emitting layer 33 b is small.

In this Example, the host material (BH1) contained in the firstlight-emitting layer 33 a has a hole mobility higher than the electronmobility, and the host material contained in the second light-emittinglayer 33 b and the host material (BCP) contained in the thirdlight-emitting layer 33 c have an electron mobility higher than the holemobility. Therefore, for transporting properties of the host materials,the hole-transporting property of the first light-emitting layer 33 a ishigh and the electron-transporting property of the second light-emittinglayer 33 b and the third light-emitting layer 33 c is high.

In the present embodiment, the first light-emitting layer 33 a is on aside of the positive electrode 2. Therefore, holes injected from thepositive electrode 2 are accepted from the hole transport layer 32through the hole injecting layer 31, and then transferred to the secondlight-emitting layer 33 b by the first light-emitting layer 33 a.

The third light-emitting layer 33 c is on a side of the negativeelectrode 4. Therefore, electrons injected from the negative electrode 4are accepted from the electron transport layer 34 through the electroninjecting layer 35, and then transferred to the second light-emittinglayer 33 b by the third light-emitting layer 33 c.

In the second light-emitting layer 33 b positioned between the firstlight-emitting layer 33 a and the third light-emitting layer 33 c, theholes transferred from the first light-emitting layer 33 a and theelectrons transferred from the third light-emitting layer 33 c aresubjected to recombination, resulting in fluorescence emission.

Since the TADF material excited in the second light-emitting layer 33 bis brought into direct contact with the host material and the TTFmaterial in the first light-emitting layer 33 a, a Dexter transitionfrom the second light-emitting layer 33 b to the first light-emittinglayer 33 a occurs. In the first light-emitting layer 33 a, TTF occursbetween the triplet excitons of the host material and the tripletexcitons of the dopant material, the energy is transferred to thesinglet excitons of the dopant, the dopant is re-excited, andfluorescence emission from the singlet excitons of the dopant occurs.

Since the thickness of the second light-emitting layer 33 b is small,Förster energy transfer from the first light-emitting layer 33 a to thethird light-emitting layer 33 c (resonance energy transfer, Förstertransition) occurs. Therefore, in the dopant of the third light-emittinglayer 33 c, fluorescence emission also occurs.

The HOMO level of BH1 and the HOMO level of BD1 are lower than the HOMOlevel of the host material and the HOMO level of the dopant material inthe second light-emitting layer 33 b. The LUMO level of BH1 and the LUMOlevel of BD1 are higher than the LUMO level of the host material and theLUMO level of the dopant material in the second light-emitting layer 33b. Therefore, holes are more easily injected into the secondlight-emitting layer 33 b than the first light-emitting layer 33 a, andelectrons are hardly leaked from the second light-emitting layer 33 b tothe first light-emitting layer 33 a.

The LUMO level of the host material and the LUMO level of the dopantmaterial in the second light-emitting layer 33 b are lower than the LUMOlevel of the host material and the LUMO level of the dopant material inthe third light-emitting layer 33 c. The HOMO level of the host materialand the HOMO level of the dopant material in the second light-emittinglayer 33 b are higher than the HOMO level of the host material and theHOMO level of the dopant material in the third light-emitting layer 33c. Therefore, electrons are more easily injected into the secondlight-emitting layer 33 b than the third light-emitting layer 33 c, andholes are hardly leaked from the second light-emitting layer 33 b to thethird light-emitting layer 33 c.

In this Example, the TTF material that emits blue light, the TADFmaterial that emits green light, and the fluorescent material that emitsred light are layered in this order. In this case, energy transfer inthe order of the T₁ level of the TADF material that emits green light,the T₁ level of the TTF material that emits blue light, and the S₁ levelof the TTF material that emits blue light occurs. Therefore, not onlygreen light emission but also blue light emission can be achieved.Further, energy transfer from the S₁ level of the TADF material thatemits green light to the S₁ level of the fluorescent material that emitsred light also occurs. Therefore, red light emission can be alsoachieved.

Accordingly, according to this Example, a highly-efficient whitelight-emitting device can be realized at low cost.

In this Example, a case where the organic EL element 10 of white lightemission using three-color light emission is formed is described as anexample, as described above. However, when the thickness of the positiveelectrode 2, and more specifically, the thickness of thelight-transmitting electrode 22 (ITO) is modified to match thewavelength of light to be enhanced, display of other color can be alsoachieved.

Example 2

In Example 1, a case where the first light-emitting layer 33 a, thesecond light-emitting layer 33 b, and the third light-emitting layer 33c are formed by doping the host material with the fluorescent materialas the dopant material is described as an example.

However, as described above, the first light-emitting layer 33 a, thesecond light-emitting layer 33 b, and the third light-emitting layer 33c may be each formed from the host material and the dopant material(fluorescent material) or from the dopant material alone.

In Example 2, the second light-emitting layer 33 b was formed from aTADF material that is a delayed fluorescent material.

Specifically, the organic EL element 10 having the followingconfiguration was formed. In Example 2, as illustrated in FIG. 2, thenon-translucent electrode 21, the light-transmitting electrode 22, thehole injecting layer 31, the hole transport layer 32, the firstlight-emitting layer 33 a, the second light-emitting layer 33 b, thethird light-emitting layer 33 c, the electron transport layer 34, theelectron injecting layer 35, and the negative electrode 4 were layeredon the substrate 1 in this order from the substrate 1 side, like Example1.

As the substrate 1, a glass substrate was used. Materials for the layerslayered on the substrate 1 and thicknesses thereof are as follows.

Non-translucent electrode 21 (positive electrode 2, reflectiveelectrode): Ag (100 nm)

Light-transmitting electrode 22 (positive electrode 2): ITO (65 nm)

Hole injecting layer 31: HAT-CN (10 nm)

Hole transport layer 32: α-NPD (20 nm)

First light-emitting layer 33 a (blue light-emitting layer,light-emitting layer 33): BH1 (host material, 90%)/BD1 (TTF material,10%) (5 nm)

Second light-emitting layer 33 b (green light-emitting layer,light-emitting layer 33): PXZ-DPS (TADF material, 10 nm)

Third light-emitting layer 33 c (red light-emitting layer,light-emitting layer 33): BCP (host material, 90%)/DCM (fluorescentmaterial, 10%) (20 nm)

Electron transport layer 34: Bphen (30 nm)

Electron injecting layer 35: LiF (1 nm)

Negative electrode 4: Ag—Mg alloy (Ag/Mg mixing ratio=0.9/0.1) (20 nm)

The organic EL element 10 in Example 2 is the same as the organic ELelement 10 in Example 1 except that the second light-emitting layer 33 bis a single film of a TADF material that is the dopant material.

Therefore, according to Example 2, the same effects as those in Example1 can be obtained. In addition, since 100% or substantially 100% of thesecond light-emitting layer 33 b is a single film of the TADF material,a decrease in efficiency of light emission generated by excitation ofexcitons by the host material and deactivation can be prevented.

Second Embodiment

Another embodiment of the disclosure will be described as follows, withreference to FIG. 3.

The present embodiment will be described about differences between thepresent embodiment and the first embodiment, and components having thesame functions as those of the components described in the firstembodiment are appended with the same reference signs, and thedescription thereof is omitted. Obviously, the same modifications asthose of the first embodiment may also be applied to the presentembodiment.

FIG. 3 is a cross-sectional view of an example of a schematicconfiguration of a light-emitting element according to the presentembodiment. In the present embodiment, the organic EL element 10 will bedescribed as an example of the light-emitting element according to thepresent embodiment.

As illustrated in FIG. 3, the organic EL element 10 according to thepresent embodiment includes the positive electrode 2 (first electrode),the organic EL layer 3, and the negative electrode 4 (second electrode)that are layered on the substrate 1 in this order, like the organic ELelement 10 according to the first embodiment.

The organic EL element 10 according to the present embodiment is abottom-emitting organic EL element, in which light emitted from thelight-emitting layer 33 of the organic EL layer 3 is extracted as whitelight from the substrate 1 side.

In the organic EL element 10 that is the bottom-emitting organic ELelement, an insulating substrate having a light-transmitting property,which is called transparent substrate or light-transmitting substrate,such as a glass substrate and a plastic substrate, is used as thesubstrate 1.

In the organic EL element 10 that is the bottom-emitting organic ELelement, it is preferable that the positive electrode 2 be formed of thelight-transmitting electrode 22 formed from a light-transmittingelectrode material and the negative electrode 4 be formed from anon-translucent electrode material such as a reflective metal electrodematerial.

As the light-transmitting electrode material and the non-translucentelectrode material, for example, the materials exemplified in the firstembodiment can be used.

The organic EL element 10 according to the present embodiment is thesame as the organic EL element 10 according to the first embodimentexcept that the bottom-emitting organic EL element is employed, thepositive electrode 2 is the light-transmitting electrode 22, and thenegative electrode 4 is formed from the non-translucent electrodematerial.

Hereinafter, the configuration of the organic EL element 10 according tothe present embodiment will be described by using Example. In thefollowing Example, a part of components will be described by usingspecific dimensions and materials as examples. However, the presentembodiment is not also limited to the following Examples.

Example 3

In Example 3, the light-transmitting electrode 22, the hole injectinglayer 31, the hole transport layer 32, the first light-emitting layer 33a, the second light-emitting layer 33 b, the third light-emitting layer33 c, the electron transport layer 34, the electron injecting layer 35,and the negative electrode 4 were layered in this order on the substrate1, as illustrated in FIG. 3.

As the substrate 1, a glass substrate was used. Materials for the layerslayered on the substrate 1 and thicknesses thereof are as follows.

Light-transmitting electrode 22 (positive electrode 2): ITO (100 nm)

Hole injecting layer 31: HAT-CN (10 nm)

Hole transport layer 32: α-NPD (20 nm)

First light-emitting layer 33 a (blue light-emitting layer,light-emitting layer 33): BH1 (host material, 90%)/BD1 (TTF material,10%) (5 nm)

Second light-emitting layer 33 b (green light-emitting layer,light-emitting layer 33): BCP (host material, 50%)/PXZ-DPS (TADFmaterial, 50%) (10 nm)

Third light-emitting layer 33 c (red light-emitting layer,light-emitting layer 33): BCP (host material, 90%)/DCM (fluorescentmaterial, 10%) (20 nm)

Electron transport layer 34: Bphen (30 nm)

Electron injecting layer 35: LiF (1 nm)

Negative electrode 4 (reflective electrode): Al (100 nm)

The organic EL element 10 in Example 3 is the same as the organic ELelement 10 in Example 1 except that the materials for the positiveelectrode 2 and the negative electrode 4 and thicknesses thereof arechanged.

In Example 1, the light emitted from the light-emitting layer 33 isextracted as white light from the negative electrode 4 side directly orafter reflection by the non-translucent electrode 21 that is areflective electrode. In Example 3, the light emitted from thelight-emitting layer 33 is extracted as white light from the substrate 1side (that is, the positive electrode 2 side) directly or afterreflection by the negative electrode 4 that is the reflective electrode.According to Example 3, the same effects as those in Example 1 can beobtained.

As described above, according to the present embodiment, thebottom-emitting organic EL element 10 having the same effects as thosein the first embodiment can be provided.

Third Embodiment

Yet another embodiment of the disclosure will be described as follows,with reference to FIGS. 4 and 5.

The present embodiment will be described about differences between thepresent embodiment and the first embodiment, and components having thesame functions as those of the components described in the firstembodiment are appended with the same reference signs, and thedescription thereof is omitted. Obviously, the same modifications asthose of the first embodiment may also be applied to the presentembodiment.

FIGS. 4 and 5 are each a cross-sectional view of an example of aschematic configuration of a light-emitting element according to thepresent embodiment and a main portion of an electronic device providedwith the light-emitting element.

The light-emitting element according to the disclosure can be developedfor display applications. Hereinafter, an organic EL display device 100provided with the organic EL element 10, as illustrated in FIGS. 4 and5, will be described as an example of an electronic device provide withthe light-emitting element according to the disclosure.

As illustrated in FIG. 5, in the organic EL display device 100 providedwith the organic EL element 10, a signal line 11 and a TFT 12 areprovided on the substrate 1 to correspond to each of pixels 100R, 100G,and 100B.

An interlayer insulating film 13 is layered as a flattened film on thesubstrate 1 over the whole region of the substrate 1 to cover the signalline 11 and the TFT 12.

For example, the signal line 11 includes a plurality of gate lines ofselecting a pixel, a plurality of source lines of writing datum, aplurality of power source lines of supplying electric power to theorganic EL element 10, and the like.

The signal line 11 is connected to an external circuit not illustrated.When an electrical signal is input in the signal line 11 from theexternal circuit, the organic EL element 10 can be driven (can be madeto emit light).

When the organic EL display device 100 is an active matrix displaydevice, at least one TFT 12 is disposed in each of the pixels 100R,100G, and 100B. In each of the pixels 100R, 100G, and 100B, a capacitorconfigured to retain a writing voltage, a compensation circuitconfigured to compensate variation of properties of the TFT 12, and thelike may be formed.

For the interlayer insulating film 13, a known photosensitive resin canbe used. As the photosensitive resin, for example, an acrylic resin or apolyimide resin can be used.

In the interlayer insulating film 13, a contact hole 13 a forelectrically connecting the positive electrode 2 in the organic ELelement 10 to the TFT 12 is provided.

An end portion of the positive electrode 2 is covered with an edge cover15. The edge cover 15 is an insulating film, and for example, includes aphotosensitive resin. The edge cover 15 prevents short circuit with thenegative electrode 4 due to concentration of the electrodes or adecrease in thickness of the organic EL layer 3, at the end portion ofthe positive electrode 2. The edge cover 15 functions as a pixelseparation film so as not to leak a current to the adjacent pixels 100R,100G, and 100B.

An opening 15 a is provided in the edge cover 15 at each of the pixels100R, 100G, and 100B. An exposed area of the positive electrode 2 due tothe opening 15 a is a light-emitting region 30 of each of the pixels100R, 100G, and 100B.

In the organic EL layer 3, the hole injecting layer 31, the holetransport layer 32, the light-emitting layer 33, the electron transportlayer 34, and the electron injecting layer 35 are provided, as describedabove.

The organic EL display device 100 illustrated in FIGS. 4 and 5 isprovided with the organic EL element 10 that emits white light as alight-emitting element. In the organic EL display device 100, theorganic EL layer 3 containing the light-emitting layer 33 is providedcontinuously over a plurality of pixels 100R, 100G, and 100B (i.e., overthe whole of the pixels).

The organic EL display device 100 is provided with a sealing body 5(sealing member) that is bonded so as to be opposite to the substrate 1,as illustrated in FIG. 5. As the sealing body 5, for example, a sealingsubstrate formed from an insulating substrate such as a glass substrateand a plastic substrate is used.

A sealing material not illustrated in the drawings is provided betweenthe substrate 1 and the sealing body 5 so as to surround a displayregion. For example, a region surrounded by the substrate 1, the sealingbody 5, and the sealing material is filled with a filling material 6.

On a surface of the sealing body 5, for example, a CF 51R is provided asa CF that transmits red light so as to correspond to the pixel 100Rconfigured to display red color, and a CF MG is provided as CF thattransmits green light so as to correspond to the pixel 100G configuredto display green color. A CF 51B is provided as a CF that transmits bluelight so as to correspond to the pixel 100B configured to display bluecolor. A black matrix (BM) 52 is provided between CFs 51R, 51G, and 51B.The BM 52 prevents entry of light from the external to each gap betweenthe CFs 51R, 51G, and 51B and leakage of light from each gap between theCFs 51R, 51G, and 51B.

When the CFs 51R, 51G, and 51B of red (R), green (G), and blue (B) areused as described above, white light can be emitted by modulating red,green, or blue color of the pixels 100R, 100G, and 100B. Therefore, evenwhen the organic EL element 10 of each of the pixels 100R, 100G, and100B emits white light, color display can be achieved.

When single light emission is used or the organic EL element 10 of eachof the pixels 100R, 100G, and 100B displays a desired color includingred, green, and blue, the CF can be omitted. When the organic EL element10 that displays a desired color and the CF are used in combination, hueshift due to improvement in color purity of each color or change inviewing angle may be suppressed.

In the present embodiment, a case where for example, the sealingsubstrate is used as the sealing body 5 is described as an example. Thepresent embodiment is not limited to the case. The sealing body 5 may bea sealing layer including an organic insulating layer, an inorganicinsulating layer, and the like.

In a case where the organic EL element 10 is developed for displayapplications, as described above, the organic EL element 10 of whitelight emission and a color filter (CF) layer are combined to select acolor of emitted light in each pixel. Thus, full color image display canbe achieved.

When the thickness of the positive electrode 2 is changed, as describedin the first embodiment, the organic EL element 10 can display a colorother than white. When a microcavity structure (optical microresonatorstructure) that express a microcavity effect is introduced into each ofthe pixels 100R, 100G, and 100B, full color image display can beachieved.

The microcavity effect is a phenomenon where emitted light is multiplyreflected between the positive electrode 2 and the negative electrode 4and resonated to make the emission spectrum steep and the emissionintensity of peak wavelength is amplified.

The distance between a reflective electrode constituting a pair ofelectrodes (i.e., the positive electrode 2 and the negative electrode 4)sandwiching the functional layer containing the light-emitting layer 33and a translucent electrode used as a light-transmitting electrode isset to match the wavelength of light to be enhanced. Thus, themicrocavity effect can be obtained. Of light emitted by thelight-emitting layer 33, a light component in which the wavelengthshifts is multiply reflected between the reflective electrode and thetranslucent electrode many times, and resonated. As a result, the lightcomponent of which the wavelength is enhanced to a desired wavelength isemitted. When the microcavity structure is introduced into each of thepixels 100R, 100G, and 100B for each color of emitted light, the opticalpath length of the organic EL element 10 in each of the pixels 100R,100G, and 100B is changed. Thus, full color image display can beachieved.

In a case of the organic EL element 10 having the microcavity structure,the light emission intensity from the organic EL element 10 and thecolor purity are improved as compared with an organic EL element havingno microcavity structure. Therefore, the light-emitting efficiency canbe enhanced.

When the microcavity structure is introduced into each of the pixels100R, 100G, and 100B, the spectrum of light emitted from the organic ELelement 10 can be adjusted by using the CF.

Examples of a method for changing the optical path length of the organicEL element 10 in each of the pixels 100R, 100G, and 100B for each colorof emitted light include a method for layering the organic EL layer 3containing the light-emitting layer 33 and the transparent electrodebetween the reflective electrode and the translucent electrode. In acase of the top-emitting organic EL element 10, examples of the methodinclude a method in which the positive electrode 2 has a layeredstructure of the non-translucent electrode 21 including a reflectiveelectrode and the light-transmitting electrode 22 including atransparent electrode and the thickness of the light-transmittingelectrode 22 on the non-translucent electrode 21 in the positiveelectrode 2 is changed for each pixel, as illustrated in FIG. 4.

In a case of the top-emitting organic EL element 10, the positiveelectrode 2 has a layered structure of the non-translucent electrode 21including a reflective electrode and the light-transmitting electrode 22including a transparent electrode, the thickness of thelight-transmitting electrode 22 is set by patterning each of the pixels100R, 100G, and 100B such that the spectrum of each color is enhanceddue to the microcavity structure, and for example, a translucent silverthin film or the like is used for the negative electrode 4 as thetranslucent electrode. Thus, the microcavity structure can be introducedinto the organic EL element 10.

Hereinafter, a case where the microcavity structure is introduced intoeach of the pixels 100R, 100G, and 100B, the CF is used in combination,and full color image display can be achieved is described as an example.The configuration of the organic EL element 10 used in the organic ELdisplay device 100 according to the present embodiment is describedspecifically by using Example. In the following Example, a part ofcomponents will be described by using specific dimensions and materialsas examples. However, the present embodiment is not also limited to thefollowing Example.

Example 4

In Example 4, the non-translucent electrode 21, the light-transmittingelectrode 22, the hole injecting layer 31, the hole transport layer 32,the first light-emitting layer 33 a, the second light-emitting layer 33b, the third light-emitting layer 33 c, the electron transport layer 34,the electron injecting layer 35, and the negative electrode 4 arelayered in this order on the substrate 1, as illustrated in FIG. 4.

As the substrate 1, a glass substrate is used. Materials for the layerslayered on the substrate 1 and thicknesses thereof are as follows.

Non-translucent electrode 21 (positive electrode 2, reflectiveelectrode): Ag (100 nm)

Light-transmitting electrode 22 (positive electrode 2): ITO (pixel 100R:30 nm, pixel 100G: 120 nm, pixel 100B: 80 nm)

Hole injecting layer 31: HAT-CN (10 nm)

Hole transport layer 32: α-NPD (20 nm)

First light-emitting layer 33 a (blue light-emitting layer,light-emitting layer 33): BH1 (host material, 90%)/BD1 (TTF material,10%) (5 nm)

Second light-emitting layer 33 b (green light-emitting layer,light-emitting layer 33): BCP (host material, 50%)/PXZ-DPS (TADFmaterial, 50%) (10 nm)

Third light-emitting layer 33 c (red light-emitting layer,light-emitting layer 33): BCP (host material, 90%)/DCM (fluorescentmaterial, 10%) (20 nm)

Electron transport layer 34: Bphen (30 nm)

Electron injecting layer 35: LiF (1 nm)

Negative electrode 4 (translucent electrode): Ag—Mg alloy (Ag/Mg mixingratio=0.9/0.1) (20 nm)

The organic EL element 10 in Example 4 is the same as the organic ELelement 10 in Example 1 except that the thickness of thelight-transmitting electrode 22 in the positive electrode 2 at each ofthe pixels 100R, 100G, and 100B is changed.

Therefore, according to Example 4, the same effects as those in Example1 can be obtained, and the microcavity structure can be introduced intoeach of the pixels 100R, 100G, and 100B.

Fourth Embodiment

Further another embodiment of the disclosure will be described asfollows, with reference to FIG. 6.

The present embodiment will be described about differences between thepresent embodiment and the first to third embodiments. Components havingthe same functions as those of the components described in the firstembodiment are appended with the same reference signs, and thedescription thereof is omitted.

FIG. 6 is a cross-sectional view of an example of a schematicconfiguration of a light-emitting element according to the presentembodiment. In the present embodiment, the organic EL element 10 will bedescribed as an example of the light-emitting element according to thepresent embodiment.

In the organic EL element 10 according to the present embodiment, thefirst light-emitting layer 33 a (first light-emitting layer), the secondlight-emitting layer 33 b (second light-emitting layer), and the thirdlight-emitting layer 33 c (third light-emitting layer) are layeredbetween the first electrode and the second electrode in this order fromthe first electrode side. In first to third embodiments, the firstelectrode is the positive electrode 2, and in the present embodiment,the first electrode is the negative electrode 4. Therefore, the organicEL element 10 according to the present embodiment is different from theorganic EL element 10 according to each of the first to thirdembodiments.

Hereinafter, the organic EL element 10 according to the presentembodiment will be described in detail as an example of the organic ELelement 10 illustrated in FIG. 6.

As illustrated in FIG. 6, the organic EL element 10 according to thepresent embodiment has a configuration in which the organic EL layer 3(organic layer or functional layer) including the first light-emittinglayer 33 a, the second light-emitting layer 33 b, and the thirdlight-emitting layer 33 c in this order from the negative electrode 4side is provided between the positive electrode 2 (second electrode) andthe negative electrode 4 (first electrode).

In the present embodiment, for example, the hole injecting layer 31 andthe hole transport layer 32 may be provided as the organic EL layer 3other than the light-emitting layer 33 between the positive electrode 2and the light-emitting layer 33, or the electron transport layer 34 andthe electron injecting layer 35 may be provided between the negativeelectrode 4 and the light-emitting layer 33.

Therefore, the organic EL element 10 according to the present embodimenthas a configuration in which the positive electrode 2, the holeinjecting layer 31, the hole transport layer 32, the thirdlight-emitting layer 33 c, the second light-emitting layer 33 b, thefirst light-emitting layer 33 a, the electron transport layer 34, theelectron injecting layer 35, and the negative electrode 4 are layered onthe substrate 1 in this order.

In this case, the first light-emitting layer 33 a containing a delayedfluorescent material having the shortest wavelength in the organic ELelement 10 may be on the negative electrode 4 side, and the thirdlight-emitting layer 33 c containing a fluorescent material having thelongest wavelength may be on the positive electrode 2 side.

In a case where the layering order of the first light-emitting layer 33a, the second light-emitting layer 33 b, and the third light-emittinglayer 33 c is reversed, as described above, there are advantages inwhich a decrease in light-emitting efficiency due to absorption of lightemitted from the first light-emitting layer 33 a into the host materialin the third light-emitting layer 33 c can be avoided.

The mobility may not necessarily satisfy the condition depending onsuitable selection of the host material. It is considered that thelayering order is reversed for solution.

In the present embodiment, it is preferable that the firstlight-emitting layer 33 a have high electron-transporting property andthe second light-emitting layer 33 b and the third light-emitting layer33 c have high hole-transporting property.

Therefore, it is preferable that the first light-emitting layer 33 acontain a material having high electron-transporting property (i.e.,material in which the electron mobility is higher than the holemobility) and the second light-emitting layer 33 b and the thirdlight-emitting layer 33 c contain a material having highhole-transporting property (i.e., material in which the hole mobility ishigher than the electron mobility).

Accordingly, when the first light-emitting layer 33 a contains the hostmaterial, it is desirable that as the host material, the material inwhich the hole mobility is higher than the electron mobility be used.

As the host material in the second light-emitting layer 33 b and thethird light-emitting layer 33 c, the host material having highhole-transporting property in which the hole mobility is higher than theelectron mobility is preferably used. As the host material, for example,a hole-transporting material such as 1,3-bis(carbazol-9-yl)benzene (mCP)is preferably used.

Electrons are efficiently injected or transported from the negativeelectrode 4 to the first light-emitting layer 33 a, and holes areefficiently injected or transported from the positive electrode 2 to thesecond light-emitting layer 33 b through the third light-emitting layer33 c. As a result, recombination of carriers is likely to occur in thevicinity of interface between the first light-emitting layer 33 a andthe second light-emitting layer 33 b. Therefore, a Dexter transitionfrom the TADF material in the second light-emitting layer 33 b to theTTF material in the first light-emitting layer 33 a is likely to occur.The Dexter transition occurs due to direct contact. However, a Förstertransition from the S₁ level of the TADF material to the S₁ level of thefluorescent material occurs when the materials are within a constantdistance without direct contact. Therefore, this configuration makes itpossible to emit lights of three colors with efficiency.

Hereinafter, the configuration of the organic EL element 10 according tothe present embodiment will be described by using Example. In thefollowing Example, a part of components will be described by usingspecific dimensions and materials as examples. However, the presentembodiment is not also limited to the following Example.

Example 5

In Example 5, the non-translucent electrode 21, the light-transmittingelectrode 22, the hole injecting layer 31, the hole transport layer 32,the third light-emitting layer 33 c, the second light-emitting layer 33b, the first light-emitting layer 33 a, the electron transport layer 34,the electron injecting layer 35, and the negative electrode 4 arelayered on the substrate 1 in this order from the substrate 1 side, asillustrated in FIG. 6.

As the substrate 1, a glass substrate is used. Materials for the layerslayered on the substrate 1 and thicknesses thereof are as follows.

Non-translucent electrode 21 (positive electrode 2, reflectiveelectrode): Ag (100 nm)

Light-transmitting 22 (positive electrode 2): ITO (65 nm)

Hole injecting layer 31: HAT-CN (10 nm)

Hole transport layer 32: α-NPD (20 nm)

Third light-emitting layer 33 c (red light-emitting layer,light-emitting layer 33): mCP (host material, 90%)/DCM (fluorescentmaterial, 10%) (20 nm)

Second light-emitting layer 33 b (green light-emitting layer,light-emitting layer 33): mCP (host material, 50%)/PXZ-DPS (TADFmaterial, 50%) (10 nm)

First light-emitting layer 33 a (blue light-emitting layer,light-emitting layer 33): BH2 (host material, 90%)/BD2 (TTF material,10%) (5 nm)

Electron transport layer 34: Bphen (30 nm)

Electron injecting layer 35: LiF (1 nm)

Negative electrode 4 (translucent electrode): Ag—Mg alloy (Ag/Mg mixingratio=0.9/0.1) (20 nm)

The organic EL element 10 in Example 5 is the same as the organic ELelement 10 in Example 1 except that the materials in the firstlight-emitting layer 33 a, the second light-emitting layer 33 b, and thethird light-emitting layer 33 c are changed and the layering order ischanged.

For the first light-emitting layer 33 a, as a TTF material and a hostmaterial, delayed fluorescent materials that cause TTF from the T₁ levelof BH2 that is the host material to the S₁ level of BD2 that is thedopant material (TTF material) are used in combination. Of thecombination of the dopant material and the host material that cause aTTF phenomenon, a host material that satisfies the conditions describedabove for carrier-transporting properties and energy levels is selected,and a dopant material suitable for the host material is selected.

The T₁ level of BH2 is lower than the T₁ level of PXZ-DPS that is thedopant material (TADF material) of the second light-emitting layer 33 b.The S₁ level of BD2 is higher than the T₁ level of BH2.

In Example 5, since the T₁ level of BH2 is lower than the T₁ level ofthe TADF material, Dexter energy transfer from the TADF material to atleast one of BH2 and BD2 occurs, and due to TTF, upconversion from theT₁ level of the BH2 or the T₁ level of BD2 to the S₁ level of BD2occurs. In the present embodiment, BD2 having a S₁ level higher than theS₁ level of the dopant material (TADF material) of the secondlight-emitting layer 33 b is made to emit light by using delayedfluorescence due to the TTF phenomenon, as described above.

At that time, the T₁ level of BD2 is higher than the T₁ level of BH2.Therefore, triplet excitons of BH2 are not transferred to BD2, tripletexcitons generated in BD2 are transferred as energy to BH2, and due tothe TTF phenomenon, the triplet excitons collide with each other on BH2with efficiency, to generate singlet excitons. The singlet excitonsgenerated due to the TTF phenomenon are transferred as energy from BH2to BD2 that emits light, and contribute to fluorescence emission of BD2.

The host material (BH2) contained in the first light-emitting layer 33 ain this Example has an electron mobility higher than the hole mobility,and the host material contained in the second light-emitting layer 33 band the host material (mCP) contained in the third light-emitting layer33 c have a hole mobility higher than the electron mobility. Therefore,for transporting properties of the host materials, the firstlight-emitting layer 33 a has high electron-transporting property andthe second light-emitting layer 33 b and the third light-emitting layer33 c have high hole-transporting property.

The LUMO Level of BH2 and the LUMO level of BD2 are higher than the LUMOlevel of the host material and the LUMO level of the dopant material inthe second light-emitting layer 33 b. The HOMO level of BH2 and the HOMOlevel of BD2 are lower than the HOMO level of the host material and theHOMO level of the dopant material in the second light-emitting layer 33b. Therefore, electrons are more easily injected into the secondlight-emitting layer 33 b than the first light-emitting layer 33 a, andelectrons are hardly leaked from the second light-emitting layer 33 b tothe first light-emitting layer 33 a.

The HOMO level of the host material and the HOMO level of the dopantmaterial in the second light-emitting layer 33 b are higher than theHOMO level of the host material and the HOMO level of the dopantmaterial in the third light-emitting layer 33 c. The LUMO level of thehost material and the LUMO level of the dopant material in the secondlight-emitting layer 33 b are lower than the LUMO level of the hostmaterial and the LUMO level of the dopant material in the thirdlight-emitting layer 33 c. Therefore, holes are more easily injectedinto the second light-emitting layer 33 b than the third light-emittinglayer 33 c, and holes are hardly leaked from the second light-emittinglayer 33 b to the third light-emitting layer 33 c.

In Example 5, each layer in the light-emitting layer 33 can be made toemit light with efficiency, like Example 1.

Fifth Embodiment

Still another embodiment of the disclosure will be described as follows,with reference to FIGS. 7 and 8.

The present embodiment will be described about differences between thepresent embodiment and the first to fourth embodiments by usingdifferences between the present embodiment and the first embodiment.Obviously, the same modifications as those of the first to fourthembodiments may be also applied to the present embodiment. Componentshaving the same functions as those of the components described in thefirst to fourth embodiments are appended with the same reference signs,and the description thereof is omitted.

FIG. 7 is a cross-sectional view of an example of a schematicconfiguration of a light-emitting element according to the presentembodiment. FIG. 8 is a view illustrating an energy diagram of each oflayers between which the first light-emitting layer (firstlight-emitting layer 33 a) is provided in the light-emitting elementaccording to the present embodiment. In the present embodiment, theorganic EL element 10 will be described as an example of thelight-emitting element according to the present embodiment.

As described above, a Dexter transition occurs only between adjacentmolecules. Therefore, as the thickness of the first light-emitting layer33 a is larger, the light-emitting efficiency is decreased. Accordingly,the thickness of the first light-emitting layer 33 a is preferably notgreater than 5 nm, and more preferably less than 5 nm.

When the first light-emitting layer 33 a is a layer in which the contentof the TTF material is high and the thickness is extremely small, aDexter transition from the T₁ level of the TADF material in the secondlight-emitting layer 33 b to the T₁ level of the TTF material in thefirst light-emitting layer 33 a is likely to occur, and light emissioncan be more efficiently achieved in the first light-emitting layer 33 a.

It is desirable that the content of the TTF material in the firstlight-emitting layer 33 a be greater than 50%. It is more desirable thatthe first light-emitting layer 33 a be formed from the dopant materialalone.

When the TTF material used in the first light-emitting layer 33 a is adelayed fluorescent material that may cause a TTF phenomenon alone, useof a dopant film that is very thin for the first light-emitting layer 33a, as illustrated in FIG. 7, can transfer excitation energy directlyfrom the TADF material in the second light-emitting layer 33 b to theTTF material (dopant material) in the first light-emitting layer 33 adue to the Dexter transition. When the first light-emitting layer 33 ais formed from the dopant material alone, a combination of the hostmaterial and the dopant material does not need to be considered.Further, loss during transfer of excitation energy between the hostmaterial and the dopant material can be decreased.

However, when the thickness of the first light-emitting layer 33 a isvery small (e.g., approximately 1 nm) such that the layer is notuniformly formed, the first light-emitting layer 33 a may not existlocally, and a functional layer layered on a side opposite to the secondlight-emitting layer 33 b with the first light-emitting layer 33 ainterposed between the functional layer and the second light-emittinglayer 33 b may be in direct contact with the second light-emitting layer33 b.

For example, in a case of the first light-emitting layer 33 a that is athin dopant film having an average thickness of approximately 0.5 nm, afilm having a thickness greater than the thickness that is targeted to amonomolecular size (e.g., in a case of film formation by vapordeposition, a film is formed by vapor deposition in a very short time,or in a case of film formation by coating, a film is formed by using asolution of extremely low concentration) is formed. In this case, eachmolecule of the TTF material is disposed in an island shape. As aresult, the first light-emitting layer 33 a is also formed in an islandshape, as illustrated in FIG. 8.

In a case of the first light-emitting layer 33 a that has an islandshape and is thin, it is necessary that carriers be trapped by aninterface between the second light-emitting layer 33 b and the firstlight-emitting layer 33 a or by an interface between the secondlight-emitting layer 33 b and a functional layer layered on the firstlight-emitting layer 33 a on a side opposite to the secondlight-emitting layer 33 b with the first light-emitting layer 33 ainterposed between the second light-emitting layer 33 b and thefunctional layer, such that the carriers are not leaked to the firstlight-emitting layer 33 a or a side of the functional layer layered onthe side to opposite to the second light-emitting layer 33 b with thefirst light-emitting layer 33 a interposed between the functional layerand the second light-emitting layer 33 b.

In the present embodiment, not only a relationship between the HOMOlevel and LUMO level of a material that is contained in the firstlight-emitting layer 33 a at least at the highest mixing ratio and theHOMO level and LUMO level of a material that is contained in the secondlight-emitting layer 33 b at least at the highest mixing ratio, but alsoa relationship between the HOMO level and LUMO level of the materialthat is contained in the second light-emitting layer 33 b at least atthe highest mixing ratio and the HOMO level and LUMO level of a materialthat is contained in the functional layer layered on the firstlight-emitting layer 33 a on the side opposite to the secondlight-emitting layer 33 b with the first light-emitting layer 33 ainterposed between the second light-emitting layer 33 b and thefunctional layer are important.

Therefore, in a case of the examples illustrated in FIGS. 7 and 8, arelationship between the HOMO level and LUMO level of the material inthe hole transport layer 32 and the HOMO level and LUMO level of thehost material in the second light-emitting layer 33 b is important forprevention of leakage of carriers to the hole transport layer 32 side.

When the first light-emitting layer 33 a is an extremely thin dopantfilm having an average thickness of approximately 0.5 nm, as describedabove, the material contained in the first light-emitting layer 33 a atthe highest mixing ratio is a TTF material.

Therefore, in the present embodiment, it is desirable that the HOMOlevel of the TTF material in the first light-emitting layer 33 a belower than the HOMO level of the material contained in the secondlight-emitting layer 33 b at least at the highest mixing ratio (e.g.,When the second light-emitting layer 33 b contains the host material,the material is the host material or the host material and the dopantmaterial, when the second light-emitting layer 33 b is formed from thedopant material alone, the material is the TADF material). It isdesirable that the LUMO level of the TTF material in the firstlight-emitting layer 33 a be higher than the LUMO level of the materialcontained in the second light-emitting layer 33 b at least at thehighest mixing ratio.

When the HOMO level of the TTF material in the first light-emittinglayer 33 a is lower than the HOMO level of the material contained in thesecond light-emitting layer 33 b at least at the highest mixing ratio,holes are hardly leaked to the first light-emitting layer 33 a side in aregion adjacent to the first light-emitting layer 33 a and the secondlight-emitting layer 33 b. Carrier can be trapped by the interfacebetween the first light-emitting layer 33 a and the secondlight-emitting layer 33 b.

When the LUMO level of the TTF material in the first light-emittinglayer 33 a is higher than the LUMO level of the material contained inthe second light-emitting layer 33 b at least at the highest mixingratio, electrons are hardly leaked to the first light-emitting layer 33a side in a region adjacent to the first light-emitting layer 33 a andthe second light-emitting layer 33 b. Carrier can be trapped by theinterface between the first light-emitting layer 33 a and the secondlight-emitting layer 33 b.

In the present embodiment, it is desirable that the HOMO level of amaterial contained in a layer layered on the first light-emitting layer33 a on the side opposite to the second light-emitting layer 33 b withthe first light-emitting layer 33 a interposed between the secondlight-emitting layer 33 b and the layer (e.g., the hole transport layer32) be lower than the HOMO level of the material contained in the secondlight-emitting layer 33 b at least at the highest mixing ratio, asillustrated in FIG. 8.

When the layer layered on the first light-emitting layer 33 a on theside opposite to the second light-emitting layer 33 b with the firstlight-emitting layer 33 a interposed between the second light-emittinglayer 33 b and the layer is, for example, the hole transport layer 32,the first light-emitting layer 33 a has an island shape, and holes arehardly leaked from the second light-emitting layer 33 b to the holetransport layer 32 in a region where the second light-emitting layer 33b is in contact with the hole transport layer 32. Carriers can betrapped by an interface between the second light-emitting layer 33 b andthe hole transport layer 32.

As illustrated in FIG. 8, it is desirable that the LUMO level of thematerial contained in the layer layered on the first light-emittinglayer 33 a on the side opposite to the second light-emitting layer 33 bwith the first light-emitting layer 33 a interposed between the secondlight-emitting layer 33 b and the layer (e.g., the hole transport layer32) be higher than the LUMO level of the material contained in thesecond light-emitting layer 33 b at least at the highest mixing ratio.

When the layer layered on the first light-emitting layer 33 a on theside opposite to the second light-emitting layer 33 b with the firstlight-emitting layer 33 a interposed between the second light-emittinglayer 33 b and the layer is, for example, the hole transport layer 32,the first light-emitting layer 33 a has an island shape, and electronsare hardly leaked from the second light-emitting layer 33 b to the holetransport layer 32 in the region where the second light-emitting layer33 b is in contact with the hole transport layer 32. Carriers can betrapped by an interface between the second light-emitting layer 33 b andthe hole transport layer 32.

Hereinafter, the configuration of the organic EL element 10 according tothe present embodiment will be described by using Example. In thefollowing Example, a part of components will be described by usingspecific dimensions and materials as examples. However, the presentembodiment is not also limited to the following Examples.

Example 6

In Example 6, the non-translucent electrode 21, the light-transmittingelectrode 22, the hole injecting layer 31, the hole transport layer 32,the third light-emitting layer 33 c, the second light-emitting layer 33b, the first light-emitting layer 33 a, the electron transport layer 34,the electron injecting layer 35, and the negative electrode 4 arelayered on the substrate 1 in this order from the substrate 1 side, asillustrated in FIG. 7.

As the substrate 1, a glass substrate is used. Materials for the layerslayered on the substrate 1 and thicknesses thereof are as follows.

Non-translucent electrode 21 (positive electrode 2, reflectiveelectrode): Ag (100 nm)

Light-transmitting 22 (positive electrode 2): ITO (65 nm)

Hole injecting layer 31: HAT-CN (10 nm)

Hole transport layer 32: α-NPD (20 nm)

First light-emitting layer 33 a (blue light-emitting layer,light-emitting layer 33): BD3 (TTF material, 0.5 nm)

Second light-emitting layer 33 b (green light-emitting layer,light-emitting layer 33): BCP (host material, 50%)/PXZ-DPS (TADFmaterial, 50%) (10 nm)

Third light-emitting layer 33 c (red light-emitting layer,light-emitting layer 33): BCP (host material, 90%)/DCM (fluorescentmaterial, 10%) (20 nm)

Electron transport layer 34: Bphen (30 nm)

Electron injecting layer 35: LiF (1 nm)

Negative electrode 4 (translucent electrode): Ag—Mg alloy (Ag/Mg mixingratio=0.9/0.1) (20 nm)

BD3 is a TTF material that has a HOMO level lower than that of BCP and aLUMO level higher than that of BCP and in which a TTF phenomenon may becaused by using a dopant material alone without a host material.

The organic EL element 10 in Example 6 is the same as the organic ELelement 10 in Example 1 except that the first light-emitting layer 33 ais an extremely thin dopant film formed from the BD3 alone.

According to the present embodiment, the effects as those in Example 1can be obtained. Further, excitation energy can be transferred directlyfrom the TADF material in the second light-emitting layer 33 b to theTTF material in the first light-emitting layer 33 a due to a Dextertransition. Therefore, loss during accepting the excitation energybetween the host material and the dopant material can be decreased.

The T₁ level of BD3 is lower than the T₁ level of PXZ-DPS that is thedopant material (TADF material) in the second light-emitting layer 33 b.Therefore, Dexter energy transfer from the dopant material (TADFmaterial) in the second light-emitting layer 33 b to BD3 occurs, andupconversion from the T₁ level to the S₁ of BD3 is caused by TTF. In thepresent embodiment, BD3 having a S₁ level higher than the S₁ level ofthe dopant material (TADF material) in the second light-emitting layer33 b is made to emit light by using delayed fluorescence due to the TTFphenomenon, as described above.

In Example 6, as the host materials in the second light-emitting layer33 b and the third light-emitting layer 33 c, the same host material asthat in Example 1 is used.

The HOMO level of BD3 is lower than the HOMO level of the host materialand the HOMO level of the dopant material in the second light-emittinglayer 33 b. The LUMO level of BD3 is higher than the LUMO level of thehost material and the LUMO level of the dopant material in the secondlight-emitting layer 33 b. Therefore, holes are more easily injectedinto the second light-emitting layer 33 b than the first light-emittinglayer 33 a, and electrons are hardly leaked from the secondlight-emitting layer 33 b to the first light-emitting layer 33 a.

The LUMO level of the host material and the LUMO level of the dopantmaterial in the second light-emitting layer 33 b are lower than the LUMOlevel of the host material and the LUMO level of the dopant material inthe third light-emitting layer 33 c. The HOMO level of the host materialand the HOMO level of the dopant material in the second light-emittinglayer 33 b are higher than the HOMO level of the host material and theHOMO level of the dopant material in the third light-emitting layer 33c. Therefore, electrons are more easily injected into the secondlight-emitting layer 33 b than the third light-emitting layer 33 c, andholes are hardly leaked from the second light-emitting layer 33 b to thethird light-emitting layer 33 c.

The HOMO level of the hole transport layer 32 (α-NPD) adjacent to thefirst light-emitting layer 33 a is lower than the HOMO level of BCP thatis the host material in the second light-emitting layer 33 b, and theLUMO level of the α-NPD is higher than the LUMO level of BCP. Therefore,carrier leakage from the second light-emitting layer 33 b to the holetransport layer 32 can be prevented or suppressed.

Sixth Embodiment

Further another embodiment of the disclosure will be described asfollows, with reference to FIG. 9.

The present embodiment will be described about differences between thepresent embodiment and the first to fifth embodiments by usingdifferences between the present embodiment and the first embodiment.Obviously, the same modifications as those of the first to fifthembodiments may also be applied to the present embodiment. The presentembodiment will be described about differences between the presentembodiment and the first to fifth embodiments, and components having thesame functions as those of the components described in the firstembodiment are appended with the same reference signs, and thedescription thereof is omitted.

FIG. 9 is a cross-sectional view of an example of a schematicconfiguration of a light-emitting element according to the presentembodiment. In the present embodiment, the organic EL element 10 will bedescribed as an example of the light-emitting element according to thepresent embodiment.

As illustrated in FIG. 9, the organic EL element 10 according to thepresent embodiment has a configuration in which a buffer layer 36(intermediate layer) containing no fluorescent material is providedbetween the second light-emitting layer 33 b and the thirdlight-emitting layer 33 c.

As described above, transfer of excitation energy between the secondlight-emitting layer 33 b and the third light-emitting layer 33 c iscaused by Förster energy transfer that does not need direct contact.Therefore, the buffer layer 36 containing no fluorescent material may beprovided between the second light-emitting layer 33 b and the thirdlight-emitting layer 33 c, as described above.

When a material for the buffer layer 36 is appropriately selected,carrier balance can be controlled by a difference of carrier (hole orelectron) mobility or the size of barrier.

Therefore, when the buffer layer 36 is provided between the secondlight-emitting layer 33 b and the third light-emitting layer 33 c, asdescribed above, the difference of carrier (hole or electron) mobilityand the size of barrier between the second light-emitting layer 33 b andthe third light-emitting layer 33 c can be changed, and the carrierbalance can be controlled.

According to the present embodiment, excitons can be certainly generatedin the second light-emitting layer 33 b. When excitons can be certainlygenerated in the second light-emitting layer 33 b, fluorescence can beemitted by the first light-emitting layer 33 a due to a Dextertransition and TTF, as described above, and fluorescence can be emittedin the third light-emitting layer 33 c due to a Förster transition.According to the present embodiment, light emission can be efficientlycaused in each layer (i.e., the first light-emitting layer 33 a, thesecond light-emitting layer 33 b, and the third light-emitting layer 33c) in the light-emitting layer 33, and the light-emitting efficiency ineach layer in the light-emitting layer 33 can be enhanced.

When the buffer layer 36 is thus provided between the secondlight-emitting layer 33 b and the third light-emitting layer 33 c, it isdesirable that the total thickness of the second light-emitting layer 33b and the buffer layer 36 be not greater than 20 nm.

A Förster transition from the S₁ level of the TADF material contained inthe second light-emitting layer 33 b to the S₁ level of the fluorescentmaterial contained in the third light-emitting layer 33 c occurs whenthe materials are within a constant distance without direct contact.When light-emitting layers having different emission peak wavelengthsare layered, excitation energy is easily transferred from thelight-emitting layer having the shorter emission peak wavelength to thelight-emitting layer having the longer emission peak wavelength. When adistance between a molecule of the TADF material contained in the secondlight-emitting layer 33 b and a molecule of the fluorescent materialcontained in the third light-emitting layer 33 c is not greater than 20nm, the Förster transition easily and certainly occurs, and the energytransfer efficiency is not deteriorated.

Hereinafter, the configuration of the organic EL element 10 according tothe present embodiment will be described by using Example. In thefollowing Example, a part of components will be described by usingspecific dimensions and materials as examples. However, the presentembodiment is not also limited to the following Example.

Example 7

In Example 7, the non-translucent electrode 21, the light-transmittingelectrode 22, the hole injecting layer 31, the hole transport layer 32,the first light-emitting layer 33 a, the second light-emitting layer 33b, the buffer layer 36, the third light-emitting layer 33 c, theelectron transport layer 34, the electron injecting layer 35, and thenegative electrode 4 are layered on the substrate 1 in this order fromthe substrate 1 side, as illustrated in FIG. 9.

As the substrate 1, a glass substrate is used. Materials for the layerslayered on the substrate 1 and thicknesses thereof are as follows.

Non-translucent electrode 21 (positive electrode 2, reflectiveelectrode): Ag (100 nm)

Light-transmitting 22 (positive electrode 2): ITO (65 nm)

Hole injecting layer 31: HAT-CN (10 nm)

Hole transport layer 32: α-NPD (20 nm)

First light-emitting layer 33 a (blue light-emitting layer,light-emitting layer 33): BH1 (host material, 90%)/BD1 (TTF material,10%) (5 nm)

Second light-emitting layer 33 b (green light-emitting layer,light-emitting layer 33): BCP (host material, 50%)/PXZ-DPS (TADFmaterial, 50%) (10 nm)

Buffer layer 36: BCP (5 nm)

Third light-emitting layer 33 c (red light-emitting layer,light-emitting layer 33): BCP (host material, 90%)/DCM (fluorescentmaterial, 10%) (20 nm)

Electron transport layer 34: Bphen (30 nm)

Electron injecting layer 35: LiF (1 nm)

Negative electrode 4 (translucent electrode): Ag—Mg alloy (Ag/Mg mixingratio=0.9/0.1) (20 nm)

As described above, the organic EL element 10 in Example 7 has the sameas the organic EL element 10 in Example 1 except that the buffer layer36 is provided between the second light-emitting layer 33 b and thethird light-emitting layer 33 c.

According to Example 7, the same effects as those in Example 1 can beobtained. Further, carrier balance between the second light-emittinglayer 33 b and the third light-emitting layer 33 c can be controlled.Therefore, excitons can be more certainly generated in the secondlight-emitting layer 33 b.

Supplement

A light-emitting element (the organic EL element 10) according to afirst aspect of the disclosure is a light-emitting element in which afunctional layer (the organic EL layer 3) containing at least a firstlight-emitting layer (the first light-emitting layer 33 a), a secondlight-emitting layer (the second light-emitting layer 33 b), and a thirdlight-emitting layer (the third light-emitting layer 33 c) is providedbetween a first electrode (one of the positive electrode 2 and thenegative electrode 4) and a second electrode (the other of the positiveelectrode 2 and the negative electrode 4). The first light-emittinglayer has the shortest emission peak wavelength of the light-emittinglayers, and contains (I) a host material and (II) a TTF material that isa delayed fluorescent material that causes a TTF phenomenon incooperation with the host material or by the TTF material alone, orcontains at least the TTF material. The second light-emitting layer islayered on the first light-emitting layer between the firstlight-emitting layer and the third light-emitting layer, and contains atleast a thermally activated delayed fluorescent material. The thirdlight-emitting layer has the longest emission peak wavelength of thelight-emitting layers, and contains at least a fluorescent material. Theexcited triplet level of at least one of the host material and the TTFmaterial contained in the first light-emitting layer is lower than theexcited triplet level of the thermally activated delayed fluorescentmaterial contained in the second light-emitting layer.

As the emission peak wavelength is shorter, the energy level of thelight-emitting material is higher. When light-emitting layers havingdifferent emission peak wavelengths are layered, excitation energy iseasily transferred from the light-emitting layer having the shorteremission peak wavelength to the light-emitting layer having the longeremission peak wavelength. In the thermally activated delayed fluorescentmaterial (TADF material), excitons can be freely transferred between theS₁ level and the T₁ level. Therefore, excitons are generated uniformlyin the S₁ level and the T₁ level of the thermally activated delayedfluorescent material. This configuration causes Förster energy transfer(Förster transition) from the thermally activated delayed fluorescentmaterial in the second light-emitting layer to the fluorescent materialin the third light-emitting layer.

The T₁ level of at least one of the host material and the TTF materialcontained in the first light-emitting layer is lower than the T₁ levelof the thermally activated delayed fluorescent material contained in thesecond light-emitting layer. Therefore, excitation energy is easilytransferred from the thermally activated delayed fluorescent material inthe second light-emitting layer to the host material or the TTF materialcontained in the first light-emitting layer. At that time, the secondlight-emitting layer is layered on the first light-emitting layer (thatis, the second light-emitting layer is in contact with the firstlight-emitting layer). Therefore, Dexter energy transfer from thethermally activated delayed fluorescent material in the secondlight-emitting layer to the host material or the TTF material in thefirst light-emitting layer occurs. Due to a TTF phenomenon,re-excitation from the T₁ level of the host material or the T₁ level ofthe TTF material to the S₁ level of the TTF material occurs in the firstlight-emitting layer.

According to this configuration, light emission can be efficientlyachieved in each of the first light-emitting layer, the secondlight-emitting layer, and the third light-emitting layer. In thelight-emitting element, the fluorescent material containing the delayedfluorescent material is used to emit light, and a phosphorescentmaterial is not used. Therefore, a reduction in cost can be attained.

A light-emitting element according to a second aspect of the disclosureis the light-emitting element according to the first aspect, whereinenergy of excitons generated in the second light-emitting layer may betransferred to the first light-emitting layer due to Dexter energytransfer, energy of excitons generated in the second light-emittinglayer may be transferred to the third light-emitting layer due toFörster energy transfer, and light emission may be caused in each of thefirst light-emitting layer, the second light-emitting layer, and thethird light-emitting layer.

According to this configuration, light emission can be efficientlyachieved in each of the first light-emitting layer, the secondlight-emitting layer, and the third light-emitting layer, as describedabove.

The light-emitting element according to a third aspect of the disclosureis the light-emitting element according to the second aspect, whereinthe Dexter energy transfer from the excited triplet level of thethermally activated delayed fluorescent material in the secondlight-emitting layer to the excited triplet level of the TTF material inthe first light-emitting layer occurs, re-excitation from the excitedtriplet level of the TTF material to the excited singlet level of theTTF material occurs, the Förster energy transfer from the excitedsinglet level of the thermally activated delayed fluorescent material inthe second light-emitting layer to the excited singlet level of thefluorescent material in the third light-emitting layer occurs.

According to this configuration, the Dexter energy transfer from theexcited triplet level of the thermally activated delayed fluorescentmaterial in the second light-emitting layer to the excited triplet levelof the TTF material in the first light-emitting layer occurs. Therefore,in the first light-emitting layer, light emission can be moreefficiently achieved. The Förster energy transfer from the excitedsinglet level of the thermally activated delayed fluorescent material inthe second light-emitting layer to the excited singlet level of thefluorescent material in the third light-emitting layer occurs.Therefore, in the third light-emitting layer, light emission can be moreefficiently achieved.

A light-emitting element according to a fourth aspect of the disclosureis the light-emitting element according to any one of the first to thirdaspects, wherein the second light-emitting layer may have a thickness ofnot greater than 20 nm.

The Förster transition from the S₁ level of the thermally activateddelayed fluorescent material contained in the second light-emittinglayer to the S₁ level of the fluorescent material contained in the thirdlight-emitting layer occurs when the materials are within a constantdistance without direct contact. When light-emitting layers havingdifferent emission peak wavelengths are layered, excitation energy iseasily transferred from the light-emitting layer having the shorteremission peak wavelength to the light-emitting layer exhibiting thelonger emission peak wavelength. When a distance between a molecule ofthe thermally activated delayed fluorescent material and a molecule ofthe fluorescent material is not greater than 20 nm, the Förstertransition easily and certainly occurs, and the energy transferefficiency is not deteriorated.

A light-emitting element according to a fifth aspect of the disclosureincludes the light-emitting element according to any one of the first tofourth aspects, wherein the first electrode may be a positive electrode,the first light-emitting layer, the second light-emitting layer, and thethird light-emitting layer may be layered in this order from the firstelectrode side, the first light-emitting layer may contain a materialhaving a hole mobility higher than the electron mobility, and the secondlight-emitting layer and the third light-emitting layer may contain amaterial having an electron mobility higher than the hole mobility.

According to this configuration, holes are efficiently injected ortransported from the first electrode to the first light-emitting layer,and electrons are efficiently injected or transported from the secondelectrode to the second light-emitting layer through the thirdlight-emitting layer. Therefore, recombination of carriers is likely tooccur in the vicinity of interface between the first light-emittinglayer and the second light-emitting layer. Accordingly, the excitongeneration probability in the second light-emitting layer is increased,and a Dexter transition from the thermally activated delayed fluorescentmaterial in the second light-emitting layer to the TTF material in thefirst light-emitting layer is likely to occur. While the Dextertransition occurs due to direct contact, a Förster transition from theS₁ level of the thermally activated delayed fluorescent material to theS₁ level of the fluorescent material occurs when the materials arewithin a constant distance without direct contact. Therefore, lights ofthree colors can be more efficiently emitted by this configuration.

A light-emitting element according to a sixth aspect of the disclosureis the light-emitting element according to any one of the first tofourth aspects, wherein the first electrode may be a negative electrode,the first light-emitting layer, the second light-emitting layer, and thethird light-emitting layer may be layered in this order from the firstelectrode side, the first light-emitting layer may contain a materialhaving an electron mobility higher than the hole mobility, and thesecond light-emitting layer and the third light-emitting layer maycontain a material having a hole mobility higher than the electronmobility.

According to this configuration, electrons are efficiently injected ortransported from the first electrode to the first light-emitting layer,and holes are efficiently injected or transported from the secondelectrode to the second light-emitting layer through the thirdlight-emitting layer. Therefore, recombination of carriers is likely tooccur in the vicinity of interface between the first light-emittinglayer and the second light-emitting layer. Accordingly, a Dextertransition from the thermally activated delayed fluorescent material inthe second light-emitting layer to the TTF material in the firstlight-emitting layer is likely to occur. While the Dexter transitionoccurs due to direct contact, a Förster transition from the S₁ level ofthe thermally activated delayed fluorescent material to the S₁ level ofthe fluorescent material occurs when the materials are within a constantdistance without direct contact. Therefore, lights of three colors canbe more efficiently emitted by this configuration.

A light-emitting element according to a seventh aspect of the disclosureis the light-emitting element according to any one of the first to sixthaspects, wherein the HOMO level of a material contained in the secondlight-emitting layer at least at the highest mixing ratio may be higherthan the HOMO level of a material contained in the first light-emittinglayer at least at the highest mixing ratio.

According to this configuration, when the positive electrode is providedon the first light-emitting layer side as the first or second electrode,holes are more easily injected into the second light-emitting layer thanthe first light-emitting layer. When the negative electrode is providedon the first light-emitting layer side as the first or second electrode,holes are hardly leaked from the second light-emitting layer to thefirst light-emitting layer. Therefore, this configuration can enhancethe exciton generation probability in the second light-emitting layer.Accordingly, the light-emitting efficiency of the second light-emittinglayer can be enhanced. In addition, due to transfer of energy ofexcitations generated in the second light-emitting layer, light emissioncan be efficiently achieved also in the first light-emitting layer andthe third light-emitting layer.

A light-emitting element according to an eighth aspect of the disclosureis the light-emitting element according to any one of the first toseventh aspects, wherein the HOMO level of the material contained in thesecond light-emitting layer at least at the highest mixing ratio may behigher than the HOMO level of a material contained in the thirdlight-emitting layer at least at the highest mixing ratio.

According to this configuration, when the positive electrode is providedon the third light-emitting layer side as the first or second electrode,holes are more easily injected into the second light-emitting layer thanthe third light-emitting layer. When the negative electrode is providedon the third light-emitting layer side as the first or second electrode,holes are hardly leaked from the second light-emitting layer to thethird light-emitting layer. Therefore, this configuration can enhancethe exciton generation probability in the second light-emitting layer.Accordingly, the light-emitting efficiency of the second light-emittinglayer can be enhanced. In addition, due to transfer of energy ofexcitations generated in the second light-emitting layer, light emissioncan be efficiently achieved also in the first light-emitting layer andthe third light-emitting layer.

A light-emitting element according to a ninth aspect of the disclosureis the light-emitting element according to any one of the first toeighth aspects, wherein the LUMO level of the material contained in thesecond light-emitting layer at least at the highest mixing ratio may belower than the LUMO level of the material contained in the firstlight-emitting layer at least at the highest mixing ratio.

According to this configuration, when the negative electrode is providedon the first light-emitting layer side as the first or second electrode,electrons are more easily injected into the second light-emitting layerthan the first light-emitting layer. When the positive electrode isprovided on the first light-emitting layer side as the first or secondelectrode, electrons are hardly leaked from the second light-emittinglayer to the first light-emitting layer. Therefore, this configurationcan enhance the exciton generation probability in the secondlight-emitting layer. Accordingly, the light-emitting efficiency of thesecond light-emitting layer can be enhanced. In addition, due totransfer of energy of excitations generated in the second light-emittinglayer, light emission can be efficiently achieved also in the firstlight-emitting layer and the third light-emitting layer.

A light-emitting element according to a tenth aspect of the disclosureis the light-emitting element according to any one of the first to ninthaspects, wherein the LUMO level of the material contained in the secondlight-emitting layer at least at the highest mixing ratio may be lowerthan the LUMO level of the material contained in the thirdlight-emitting layer at least at the highest mixing ratio.

According to this configuration, when the negative electrode is providedon the third light-emitting layer side as the first or second electrode,electrons are more easily injected into the second light-emitting layerthan the third first light-emitting layer. When the positive electrodeis provided on the third light-emitting layer side as the first orsecond electrode, electrons are hardly leaked from the secondlight-emitting layer to the third light-emitting layer. Therefore, thisconfiguration can enhance the exciton generation probability in thesecond light-emitting layer. Accordingly, the light-emitting efficiencyof the second light-emitting layer can be enhanced. In addition, due totransfer of energy of excitations generated in the second light-emittinglayer, light emission can be efficiently achieved also in the firstlight-emitting layer and the third light-emitting layer.

A light-emitting element according to an eleventh aspect of thedisclosure is the light-emitting element according to any one of thefirst to tenth aspects, wherein the first light-emitting layer may havea thickness of less than 5 nm, and the content of the TTF material inthe first light-emitting layer may be greater than 50%.

In this configuration, the first light-emitting layer is an extremelythin layer containing the TTF material at a high content. Therefore, theDexter energy transfer from the excited triplet level of the thermallyactivated delayed fluorescent material in the second light-emittinglayer to the excited triplet level of the TTF material in the firstlight-emitting layer is likely to occur. Therefore, light emission canbe more efficiently achieved in the first light-emitting layer.

A light-emitting element according to a twelfth aspect of the disclosureis the light-emitting element according the eleventh aspect, wherein thefunctional layer may contain at least a layer layered on a side oppositeto the second light-emitting layer with the first light-emitting layerinterposed between the second light-emitting layer and the functionallayers, and the HOMO level of a material contained in the layer on thefirst light-emitting layer, of the layer layered on a side opposite tothe second light-emitting layer with the first light-emitting layerinterposed between the second light-emitting layer and the functionallayers, may be lower than the HOMO level of the material contained inthe second light-emitting layer at least at the highest mixing ratio.

When the thickness of the first light-emitting layer is small, the firstlight-emitting layer has an island shape. The first light-emitting layermay be in direct contact with a layer layered on a side opposite to thesecond light-emitting layer with the first light-emitting layerinterposed between the second light-emitting layer and the layer.However, according to this configuration, holes are hardly leaked fromthe second light-emitting layer to the layer layered on the sideopposite to the second light-emitting layer with the firstlight-emitting layer interposed between the second light-emitting layerand the layer. Carriers can be trapped by an interface between thesecond light-emitting layer and the layer layered on the side oppositeto the second light-emitting layer with the first light-emitting layerinterposed between the second light-emitting layer and the layer.

A light-emitting element according to a thirteenth aspect of thedisclosure is the light-emitting element according the eleventh aspect,wherein the functional layer may contain at least a layer layered on aside opposite to the second light-emitting layer with the firstlight-emitting layer interposed between the second light-emitting layerand the functional layers, and the LUMO level of a material contained inthe layer on the first light-emitting layer, of the layer layered on aside opposite to the second light-emitting layer with the firstlight-emitting layer interposed between the second light-emitting layerand the functional layers, may be higher than the LUMO level of thematerial contained in the second light-emitting layer at least at thehighest mixing ratio.

When the thickness of the first light-emitting layer is small, the firstlight-emitting layer has an island shape. The first light-emitting layermay be in direct contact with a layer layered on a side opposite to thesecond light-emitting layer with the first light-emitting layerinterposed between the second light-emitting layer and the layer.However, according to this configuration, electrons are hardly leakedfrom the second light-emitting layer to the layer layered on the sideopposite to the second light-emitting layer with the firstlight-emitting layer interposed between the second light-emitting layerand the layer. Carriers can be trapped by an interface between thesecond light-emitting layer and the layer layered on the side oppositeto the second light-emitting layer with the first light-emitting layerinterposed between the second light-emitting layer and the layer.

A light-emitting element according to a fourteenth aspect of thedisclosure is the light-emitting element according to any one of thefirst to thirteenth aspects, wherein a buffer layer containing nofluorescent material may be provided between the second light-emittinglayer and the third light-emitting layer.

Transfer of excitation energy between the second light-emitting layerand the third light-emitting layer is caused by Förster energy transferthat does not need direct contact. Therefore, the buffer layercontaining no fluorescent material may be provided between the secondlight-emitting layer and the third light-emitting layer.

When the buffer layer is provided, a difference of carrier (hole orelectron) mobility and the size of barrier between the secondlight-emitting layer and the third light-emitting layer can be changed,and the carrier balance can be controlled. According to thisconfiguration, excitons can be certainly generated in the secondlight-emitting layer. Therefore, the light-emitting efficiency of eachlayer can be enhanced.

A light-emitting element according to a fifteenth aspect of thedisclosure is the light-emitting element according to fourteenth aspect,wherein the total thickness of the second light-emitting layer and thebuffer layer may be not greater than 20 nm.

A Förster transition from the S₁ level of the thermally activateddelayed fluorescent material contained in the second light-emittinglayer to the S₁ level of the fluorescent material contained in the thirdlight-emitting layer occurs when the materials are within a constantdistance without direct contact. When light-emitting layers havingdifferent emission peak wavelengths are layered, excitation energy iseasily transferred from the light-emitting layer having the shorteremission peak wavelength to the light-emitting layer exhibiting thelonger emission peak wavelength. When a distance between a molecule ofthe thermally activated delayed fluorescent material and a molecule ofthe fluorescent material is not greater than 20 nm, the Förstertransition easily and certainly occurs, and the energy transferefficiency is not deteriorated. When the buffer layer is providedbetween the second light-emitting layer and the third light-emittinglayer, it is preferable that the total thickness of the secondlight-emitting layer and the buffer layer be not greater than 20 nm.

A light-emitting element according to a sixteenth aspect of thedisclosure is the light-emitting element according to any one of thefirst to fifteenth aspects, wherein the first light-emitting layer maybe a blue light-emitting layer, the second light-emitting layer may be agreen light-emitting layer, and the third light-emitting layer may be ared light-emitting layer.

According to this configuration, each of the blue light-emitting layer,the green light-emitting layer, and the red light-emitting layer can bemade to efficiently emit light. Therefore, a white light-emittingelement having high light-emitting efficiency can be provided.

A light-emitting element according to a seventeenth aspect of thedisclosure may be an organic EL element that is the light-emittingelement according to any one of the first to sixteenth aspects.

According to this configuration, an organic EL element having highlight-emitting efficiency can be provided.

An electronic device (the organic EL display device 100) according to aneighteenth aspect of the disclosure may be provided with thelight-emitting element (the organic EL element 10) according to any oneof the first to seventeenth aspects.

According to this configuration, an electronic device provided with alight-emitting element having high light-emitting efficiency can beprovided.

An electronic device according to a nineteenth aspect of the disclosuremay be an illumination device that is the electronic device according tothe eighteenth aspect.

According to this configuration, an illumination device having highlight-emitting efficiency can be provided.

An electronic device according to a twentieth aspect of the disclosuremay be a display device that is the electronic device according to anyone of the first to seventh aspects.

According to this configuration, a display device having highlight-emitting efficiency can be provided.

A method for manufacturing a light-emitting element (the organic ELelement 10) according to a twenty-first aspect of the disclosureincludes forming a functional layer (the organic EL layer 3) containingat least a light-emitting layer (the light-emitting layer 33) between afirst electrode (one of the positive electrode 2 and the negativeelectrode 4) and a second electrode (the other of the positive electrode2 and the negative electrode 4). In the method, the forming thefunctional layer includes forming a first light-emitting layer (thefirst light-emitting layer 33 a), forming a second light-emitting layer(the second light-emitting layer 33 b), and forming a thirdlight-emitting layer (the third light-emitting layer 33 c). The firstlight-emitting layer has the shortest emission peak wavelength of thelight emitting layers and contains (I) a host material and (II) a TTFmaterial that is a delayed fluorescent material that causes a TTFphenomenon in cooperation with the host material or by the TTF materialalone, or at least the TTF material. The second light-emitting layercontains at least a thermally activated delayed fluorescent material,and the excited triplet level of the thermally activated delayedfluorescent material is higher than the excited triplet level of atleast one of the host material and the TTF material contained in thefirst light-emitting layer. The third light-emitting layer has thelongest emission peak wavelength of the light emitting layers andcontains at least a fluorescent material. The formation of the firstlight-emitting layer and the formation of the second light-emittinglayer are continuously performed such that the second light-emittinglayer is layered between the first light-emitting layer and the thirdlight-emitting layer and the first light-emitting layer and the secondlight-emitting layer are adjacent to each other.

According to the method, the same effects as those in the first aspectcan be obtained.

A light emission method according to a twenty-second aspect of thedisclosure includes transferring the energy of excitons generated in asecond light-emitting layer (the second light-emitting layer 33 b)containing at least a thermally activated delayed fluorescent materialto a first light-emitting layer (the first light-emitting layer 33 a) byDexter energy transfer, transferring the energy of excitons generated inthe second light-emitting layer to a third light-emitting layer (thethird light-emitting layer 33 c) by Förster energy transfer to make thefirst, second, and third light-emitting layers to emit light. The firstlight-emitting layer is layered on the second light-emitting layer, hasan emission peak wavelength shorter than an emission peak wavelength ofthe second light-emitting layer, and contains a host material and a TTFmaterial that is a delayed fluorescent material that causes a TTFphenomenon in cooperation with the host material or by the TTF materialalone, or at least the TTF material, the excited triplet level of atleast one of the host material and the TTF material is lower than theexcited triplet level of the thermally activated delayed fluorescentmaterial. The third light-emitting layer is layered on the secondlight-emitting layer on a side opposite to the first light-emittinglayer, has an emission peak wavelength longer than an emission peakwavelength of the second light-emitting layer, and contains at least thefluorescent material.

According to the method, the same effects as those in the second aspectcan be obtained.

The light emission method according to a twenty-third aspect of thedisclosure may be the light emission method according to thetwenty-second aspect, wherein the Dexter energy transfer from theexcited triplet level of the thermally activated delayed fluorescentmaterial in the second light-emitting layer to the excited triplet levelof the TTF material in the first light-emitting layer occurs,re-excitation from the excited triplet level of the TTF material to theexcited singlet level of the TTF material occurs, the Förster energytransfer from the excited singlet level of the thermally activateddelayed fluorescent material in the second light-emitting layer to theexcited singlet level of the fluorescent material in the thirdlight-emitting layer occurs, and light emission is achieved in each ofthe first light-emitting layer, the second light-emitting layer, andthird light-emitting layer.

According to the method, the same effects as those in the third aspectcan be obtained.

REFERENCE SIGNS LIST

-   1 Substrate-   2 Positive electrode-   3 Organic EL layer (functional layer)-   4 Negative electrode-   5 Sealing body-   6 Filling material-   10 Organic EL element-   11 Signal line-   12 TFT-   13 Interlayer insulating film-   13 a Contact hole-   15 Edge cover-   15 a Opening-   21 Non-translucent electrode-   22 Light-transmitting electrode-   30 Light-emitting region-   31 Hole injecting layer-   32 Hole transport layer-   33 Light-emitting layer-   33 a First light-emitting layer-   33 b Second light-emitting layer-   33 c Third light-emitting layer-   34 Electron transport layer-   35 Electron injecting layer-   36 Buffer layer-   51R, 51G, 51B CF-   100 Organic EL display device

The invention claimed is:
 1. A light-emitting element comprising: afirst electrode; a second electrode; and a functional layer containingat least a first light-emitting layer, a second light-emitting layer,and a third light-emitting layer, the functional layer being disposedbetween the first electrode and the second electrode, wherein the firstlight-emitting layer has a shortest emission peak wavelength of thelight-emitting layers, and contains a host material and atriplet-triplet-fusion (TTF) material or at least the TTF material, theTTF material being a delayed fluorescent material that causes a TTFphenomenon by the TTF material alone or in cooperation with the hostmaterial, the second light-emitting layer is layered on the firstlight-emitting layer between the first light-emitting layer and thethird light-emitting layer, and contains at least a thermally activateddelayed fluorescent material, the thermally activated delayedfluorescent material being a delayed fluorescent material in whichreverse intersystem crossing occurs from an excited triplet level to anexcited singlet level by thermal activation, the third light-emittinglayer has a longest emission peak wavelength of the light-emittinglayers, and contains at least a fluorescent material, and an excitedtriplet level of at least one of the host material and the TTF materialcontained in the first light-emitting layer is lower than the excitedtriplet level of the thermally activated delayed fluorescent materialcontained in the second light-emitting layer.
 2. The light-emittingelement according to claim 1, wherein energy of excitons generated inthe second light-emitting layer is transferred to the firstlight-emitting layer due to a Dexter energy transfer, energy of excitonsgenerated in the second light-emitting layer is transferred to the thirdlight-emitting layer due to a Förster energy transfer, and each of thefirst light-emitting layer, the second light-emitting layer, and thethird light-emitting layer emits light.
 3. The light-emitting elementaccording to claim 2, wherein the Dexter energy transfer occurs from theexcited triplet level of the thermally activated delayed fluorescentmaterial in the second light-emitting layer to an excited triplet levelof the TTF material in the first light-emitting layer, re-excitationoccurs from the excited triplet level of the TTF material to an excitedsinglet level of the TTF material, and the Förster energy transferoccurs from the excited singlet level of the thermally activated delayedfluorescent material in the second light-emitting layer to an excitedsinglet level of the fluorescent material in the third light-emittinglayer.
 4. The light-emitting element according to claim 1, wherein thesecond light-emitting layer has a thickness of not greater than 20 nm.5. The light-emitting element according to claim 1, wherein the firstelectrode includes a positive electrode, the first light-emitting layer,the second light-emitting layer, and the third light-emitting layer arelayered in this order from the first electrode side, the firstlight-emitting layer contains a material having a hole mobility higherthan the electron mobility, and the second light-emitting layer and thethird light-emitting layer contain a material having an electronmobility higher than the hole mobility.
 6. The light-emitting elementaccording to claim 1, wherein the first electrode is a negativeelectrode, the first light-emitting layer, the second light-emittinglayer, and the third light-emitting layer are layered in this order fromthe first electrode side, the first light-emitting layer contains amaterial having an electron mobility higher than the hole mobility, andthe second light-emitting layer and the third light-emitting layercontain a material having a hole mobility higher than the electronmobility.
 7. The light-emitting element according to claim 1, wherein ahighest occupied molecular orbital (HOMO) level of a material containedin the second light-emitting layer at least at a highest mixing ratio ishigher than a HOMO level of a material contained in the firstlight-emitting layer at least at the highest mixing ratio.
 8. Thelight-emitting element according to claim 1, wherein a highest occupiedmolecular orbital (HOMO) level of the material contained in the secondlight-emitting layer at least at a highest mixing ratio is higher than aHOMO level of a material contained in the third light-emitting layer atleast at the highest mixing ratio.
 9. The light-emitting elementaccording to claim 1, wherein a lowest unoccupied molecular orbital(LUMO) level of the material contained in the second light-emittinglayer at least at a highest mixing ratio is lower than a LUMO level ofthe material contained in the first light-emitting layer at least at thehighest mixing ratio.
 10. The light-emitting element according to claim1, wherein a lowest unoccupied molecular orbital (LUMO) level of thematerial contained in the second light-emitting layer at least at ahighest mixing ratio is lower than a LUMO level of the materialcontained in the third light-emitting layer at least at the highestmixing ratio.
 11. The light-emitting element according to claim 1,wherein the first light-emitting layer has a thickness of less than 5nm, and a content of the TTF material in the first light-emitting layeris greater than 50%.
 12. The light-emitting element according to claim11, wherein the functional layer contains at least a layer layered on aside opposite to the second light-emitting layer with the firstlight-emitting layer interposed between the second light-emitting layerand the functional layers, and a highest occupied molecular orbital(HOMO) level of a material contained in the layer on the firstlight-emitting layer, of the layer layered on a side opposite to thesecond light-emitting layer with the first light-emitting layerinterposed between the second light-emitting layer and the functionallayers, is lower than a HOMO level of the material contained in thesecond light-emitting layer at least at a highest mixing ratio.
 13. Thelight-emitting element according to claim 11, wherein the functionallayer contains at least a layer layered on a side opposite to the secondlight-emitting layer with the first light-emitting layer interposedbetween the second light-emitting layer and the functional layers, and alowest unoccupied molecular orbital (LUMO) level of a material containedin the layer on the first light-emitting layer, of the layer layered ona side opposite to the second light-emitting layer with the firstlight-emitting layer interposed between the second light-emitting layerand the functional layers, is higher than a LUMO level of the materialcontained in the second light-emitting layer at least at a highestmixing ratio.
 14. The light-emitting element according to claim 1,further comprising: a buffer layer containing no fluorescent materialbetween the second light-emitting layer and the third light-emittinglayer.
 15. The light-emitting element according to claim 14, wherein atotal thickness of the second light-emitting layer and the buffer layeris not greater than 20 nm.
 16. A method for manufacturing alight-emitting element comprising: forming a functional layer containingat least a light-emitting layer between a first electrode and a secondelectrode, wherein the forming of the functional layer includes forminga first light-emitting layer, forming a second light-emitting layer, andforming a third light-emitting layer, the first light-emitting layer hasa shortest emission peak wavelength of the light-emitting layers andcontains a host material and a triplet-triplet-fusion (TTF) material orat least the TTF material, the TTF material being a delayed fluorescentmaterial that causes a TTF phenomenon by the TTF material alone or incooperation with the host material, the second light-emitting layercontains at least a thermally activated delayed fluorescent material,the thermally activated delayed fluorescent material being a material inwhich reverse intersystem crossing occurs from an excited triplet levelto an excited singlet level by thermal activation, the excited tripletlevel of the thermally activated delayed fluorescent material is higherthan an excited triplet level of at least one of the host material andthe TTF material contained in the first light-emitting layer, the thirdlight-emitting layer has a longest emission peak wavelength of thelight-emitting layers and contains at least a fluorescent material, andthe forming of the first light-emitting layer and the forming of thesecond light-emitting layer are continuously performed such that thesecond light-emitting layer is layered between the first light-emittinglayer and the third light-emitting layer and the first light-emittinglayer and the second light-emitting layer are adjacent to each other.17. A light emission method comprising: transferring energy of excitonsgenerated in a second light-emitting layer containing at least athermally activated delayed fluorescent material to a firstlight-emitting layer by Dexter energy transfer, the thermally activateddelayed fluorescent material being a delayed fluorescent material inwhich reverse intersystem crossing occurs from an excited triplet levelto an excited singlet level by thermal activation; and transferring theenergy of excitons generated in the second light-emitting layer to athird light-emitting layer by Förster energy transfer to make the firstlight-emitting layer, the second light-emitting layer, and the thirdlight-emitting layer to emit light, wherein the first light-emittinglayer is layered on the second light-emitting layer, has an emissionpeak wavelength shorter than an emission peak wavelength of the secondlight-emitting layer, and contains a host material and atriplet-triplet-fusion (TTF) material or at least the TTF material, theTTF material being a delayed fluorescent material that causes a TTFphenomenon by the TTF material alone or in cooperation with the hostmaterial, an excited triplet level of at least one of the host materialand the TTF material is lower than the excited triplet level of thethermally activated delayed fluorescent material, and the thirdlight-emitting layer is layered on the second light-emitting layer on aside opposite to the first light-emitting layer, has an emission peakwavelength longer than an emission peak wavelength of the secondlight-emitting layer, and contains at least the fluorescent material.18. The light emission method according to claim 17, wherein the Dexterenergy transfer occurs from the excited triplet level of the thermallyactivated delayed fluorescent material in the second light-emittinglayer to an excited triplet level of the TTF material in the firstlight-emitting layer, re-excitation occurs from the excited tripletlevel of the TTF material to an excited singlet level of the TTFmaterial, the Förster energy transfer occurs from the excited singletlevel of the thermally activated delayed fluorescent material in thesecond light-emitting layer to an excited singlet level of thefluorescent material in the third light-emitting layer, and each of thefirst light-emitting layer, the second light-emitting layer, and thethird light-emitting layer emits light.